Preheat your oven to 400°F (205°C).
Cook the mini raviolis until just al dente - about 1 minute less than the package recommends. They’ll finish cooking in the oven. Skip this step if using ravioles.
Once they are done, drain them and coat them with enough olive oil so they do not stick together.
Spread them on a plate or tray to cool slightly while you prepare the rest of the dish.
If using ravioles, you can skip the boiling step completely. It will cook in the oven.
Prepare zucchini by slicing them into ⅛ inch slices with a knife or a mandoline slicer, pat to dry.
For the salmon filets, remove any skin that is on them and then cut into 1 inch cubes - this ensures they are cooked evenly while baking.
Grate the white cheese using the large holes of a box grater.
Once all your ingredients are prepped, in a heavy large skillet or large baking dish, assemble the ingredients in the following layers: zucchini slices, ravioles or mini ravioli, ½ the salmon cubes, followed by creme fraiche. After each layer of creme fraiche, sprinkle a pinch of salt, pepper, and a tiny bit of nutmeg.
Make sure you go in this order because you want the creme fraiche to be on top of the salmon while baking. This keeps it from drying out during the cooking process.
Repeat layers one more time using up the remaining half of the salmon cubes.
After the last layer, you should end up with creme fraiche on top. Do one additional layer of zucchini slices with whatever is remaining. Don't worry if this layer of zucchini doesn't cover the dish evenly.
Top the entire dish with the grated cheese of your choice (gruyere, white cheddar, parmesan, etc.)
Bake for 40-45 minutes until golden brown on top and bubbling.
Turn on your broiler to high for 2-3 minutes to further brown the top layer of cheese if you’d like to add more flavor and texture to your dish. Usually, it would come out already browned enough so this step is only if you'd like to brown the cheese layer some more.
It will come out piping hot so let it cool for at least 20 minutes to let the dish set a bit before serving. Serving too soon out of the oven results in it not holding its structure.