Powershell nested JSON to csv conversion

I suggest collecting the property-name-value pairs iteratively in an ordered hashtable ([ordered] @{}), which can then be cast to [pscustomobject] to convert it to a custom object.

No property names are hard-coded in the following solution, but the object-graph structure is assumed to follow the pattern in your sample JSON, which is limited to one level of nesting - if you need to process arbitrarily nested objects, this answer may be a starting point.

Reflection (discovery of the property names and values) is performed via the intrinsic .psobject property that PowerShell makes available on all objects.

# Parse sample JSON into an array of [pscustomobject] graphs.
$fromJson = ConvertFrom-Json @'
[
  {"metadata":{"systemId":"da1895","legalEntity":"A0"},"recordContent":{"positionDate":"2019-04-08 00:00:00.0","account":{"string":"G32"},"seg":{"string":"S"},"strike":{"double":4.4}}}
  ,
  {"metadata":{"systemId":"45364d","legalEntity":"5G"},"recordContent":{"positionDate":"2019-04-08 00:00:00.0","account":{"string":"G81"},"seg":{"string":"S"},"strike":{"double":5.0}}}
]
'@

# Initialize an aux. ordered hashtable to collect the property-name-value
# pairs in.
$oht = [ordered] @{}

$fromJson | ForEach-Object {
    $oht.Clear()
    # Loop over top-level properties.
    foreach ($topLevelProp in $_.psobject.Properties) {
      # Loop over second-level properties.
      foreach ($prop in $topLevelProp.Value.psobject.Properties) {
        if ($prop.Value -is [System.Management.Automation.PSCustomObject]) {
          # A nested value: Use the value of the (presumed to be one-and-only)
          #                 property of the object stored in the value.
          $oht[$prop.Name] = $prop.Value.psobject.Properties.Value
        }
        else {
          # A non-nested value: use as-is.
          $oht[$prop.Name] = $prop.Value
        }
      }
    }
    # Construct and output a [pscustomobject] from the aux. ordered hashtble.
    [pscustomobject] $oht
  } |
      ConvertTo-Csv  # Replace this with Export-Csv to export to a file.

The above yields:

"systemId","legalEntity","positionDate","account","seg","strike"
"da1895","A0","2019-04-08 00:00:00.0","G32","S","4.4"
"45364d","5G","2019-04-08 00:00:00.0","G81","S","5"

A few years ago, I wrote a reusable Flatten-Object function for this.
The only difference is that it combines the (sub)property names with the parent property names as they might not be unique:

$JsonParameters |Flatten-Object |Format-Table

metadata.systemId metadata.legalEntity recordContent.positionDate recordContent.account.string recordContent.seg.string recordContent.strike.double
----------------- -------------------- -------------------------- ---------------------------- ------------------------ ---------------------------
da1895            A0                   2019-04-08 00:00:00.0      G32                          S                                                4.4
45364d            5G                   2019-04-08 00:00:00.0      G81                          S                                                  5

Try this:

$data = ConvertFrom-Json @"
[
{"metadata":{"systemId":"da1895","legalEntity":"A0"},"recordContent":{"positionDate":"2019-04-08 00:00:00.0","account":{"string":"G32"},"seg":{"string":"S"},"strike":{"double":4.4}}},
{"metadata":{"systemId":"45364d","legalEntity":"5G"},"recordContent":{"positionDate":"2019-04-08 00:00:00.0","account":{"string":"G81"},"seg":{"string":"S"},"strike":{"double":5.0}}}
]
"@

$data | Select-Object -Property @{l="systemId"; e={$_.metadata.systemId}}, @{l="legalEntity"; e={$_.metadata.legalEntity}},
@{l="positionDate"; e={$_.recordContent.positionDate}}, @{l="account"; e={$_.recordContent.account.string}},
@{l="seg"; e={$_.recordContent.seg.string}}, @{l="strike"; e={$_.recordContent.strike.double}} | Export-Csv

This should work with any nested psobject.

$json = @'
{"metadata":{"systemId":"da1895","legalEntity":"A0"},"recordContent":{"positionDate":"2019-04-08 00:00:00.0","account":{"string":"G32"},"seg":{"string":"S"},"strike":{"double":4.4}}}

'@
$obj = ConvertFrom-Json $json


$obj.recordContent | gm -MemberType NoteProperty | % {
    $prop = $_.name
    if ($obj.recordContent.$prop.GetType().name -eq 'pscustomobject') {
        $obj.recordContent.$prop = $obj.recordContent.$prop.psobject.Members | where membertype -eq noteproperty | select -ExpandProperty value
    }
    $obj.metadata | add-member -MemberType NoteProperty -Name $prop -Value $obj.recordContent.$prop

}
$newobj = $obj.metadata
$newobj