Make the roux: melt the butter in a heavy-bottomed saucepan.
Stir in the flour and cook, stirring constantly, until the paste cooks and bubbles a bit, but don't let it brown—about 2 minutes. It should smell like shortbread cookies when it’s ready.
Add the milk, bit at a time to avoid clumps. If you have clumps, use a whisk to work them through.
Continue to stir as the sauce thickens.
Bring it to a boil.
Add salt, nutmeg and pepper to taste, lower the heat and cook, stirring for 2 to 3 minutes more.
Remove from the heat. To cool this sauce for later use, cover it with parchment paper or saran wrap to prevent a skin from forming.
If you want to add cheese: Take off the heat and stir in 1/2 cup grated cheese (for this I suggest parmesan, grano padano, gruyère or a nice comté). If the cheese isn’t completely melted, put back on a low heat until fully melted.
Clean the mushrooms thoroughly from dirt, twigs, leaves etc. If mushrooms are very large, give them a rough chop.
Put a good couple tablespoons of olive oil or butter (or both) in a sautée pan on medium to high heat.
Once warmed, add the mushrooms (they should sizzle).
Add salt and pepper and cook until most of the water has evaporated.
Add the garlic and cook for a few minutes.
Then add the wine and cook until all liquid has evaporated.
Turn down heat and add crème fraîche.
Add the fresh herbs at the end and adjust the seasoning. You can use any herbs you like, but I’ve had rave reviews about the tarragon!