Cut the mirlitons in half from the stem end to the blossom end.
Remove the seed
Toss the 8 halves in a mixing bowl with olive oil, salt and pepper to coat and season.
Cook in a 350F oven on a sheet pan, cut side up until just tender. Reserve.
Sweat the onion in the butter in a heavy saute pan until soft.
Add the sausage and cook for a few more minutes
Add the shrimp and cook until just under done.
Combine with the parsley and cracker crumb in a mixing bowl. Season with salt and pepper. Reserve the mixture.
Reduce the cream by one third over low heat
Whisk in mustard and adjust seasoning with lemon juice, salt and pepper. Hold in a warm place.
Divide the stuffing by eight and heap each portion on each of the 8 mirliton halves.
Bake in a 350F oven on a sheet pan, until the stuffing is golden and the item is fully heated through.
Plate each mirliton half and drizzle with creole mustard sauce.
Garnish with green onion if desired.