First, make the Pâte à Choux dough by melting the butter in the water with salt.
Once the butter has almost completely melted, add the flour and stir until it has transformed into a dough.
Add the dough to a stand mixer (or alternatively to a large bowl) and let sit for at least 3 minutes. Stir occasionally to release heat.
While the dough cools, start in on the Mornay Sauce by melting the butter over medium heat.
Once the butter has melted, add the flour and stir in the oat milk (or regular milk, depending on preference). Add the milk in small amounts and stir to avoid clumps or any browning of the sauce.
Let the Mornay Sauce rest (no heat) and return to the Pâte à Choux by adding the eggs, one at a time, while you put the stand mixer somewhere between low and medium speed (or while you're stirring the dough by hand).
Continuing mixing the Pâte à Choux and add the dijon mustard while stirring
Don't beat the dough too much; just enough so it's smooth. Then cover the dough with a kitchen towel and set aside.
Returning to the Mornay Sauce, keep it off the heat and add salt, cayenne pepper, and gruyere or other swiss-style cheese. Stir until mixed and creamy.
Prep a shallow baking dish by rubbing butter all along its surface. Cover the bottom of the dish with a thin layer of parmesan cheese. Pour 1 cup of the Mornay Sauce atop the cheese and spread evenly across the bottom of the dish. Add sauteed greens to the layer of Mornay Sauce (optional).
With an ice cream scooper in one hand and a spoon in the other, remove one scoop of your Pâte à Choux with the former while shaping and releasing it into a pot of boiling water with your spoon. Poach about 8 gnocchis at a time for 2 minutes, laying them to rest on a paper towel-covered plate once done.
Add the gnocchi to the baking dish (note: they aren't fully cooked yet!) and cover them with the rest of your Mornay Sauce. Finish with the remainder of your parmesan cheese.
Put the baking dish on a tray and put it into the oven at 350 F for 15 minutes. Put it back in at 400 for another 15-20 minutes until a nice brown layer has formed on top of the dish. Bon Appétit!