Best-Ever Mushroom Sauce

I've been making this best-ever mushroom sauce for years now and I'd never get sick of it even if I had to eat it everyday. It's a absolutely delicious dressing to put on mashed potatoes, lightly-steamed vegetables, or any grain, just as long as you can stop yourself from eating it right out of the pan, but my favourite way to eat it is as a topping for quinoa-oat croquettes.

Best-ever mushroom sauce

4 tablespoons butter
2 medium onions, chopped
1 1b. white mushrooms, or with some creminis, sliced
1 cup yogurt
juice of 2 lemons
2 tablespoons fresh parsley, chopped
1/2 teaspoon dried mustard
1 teaspoon sea salt
1 tablespoon unbleached all-purpose flour (optional)
fresh ground black pepper

Melt butter in a large saucepan or wok over medium heat. Add onions and cook for 8 minutes or until they are soft and translucent. Stir in the mushrooms, cover with a tight fitting lid, and cook for 10 minutes or until the mushrooms are reduced but still slightly plump.

Remove the lid and stir in the yogurt, lemon juice, parsley, mustard and salt. Reduce the heat to medium-low and simmer the sauce without boiling, stirring occasionally, for at least 30 minutes — or longer if you want a thicker sauce. If you want to speed up the process, stir in a tablespoon of unbleached all-purpose flour a few minutes after adding the yogurt and other ingredients and simmer for 15 minutes, stirring frequently.

Before serving, add fresh ground black pepper to taste and stir. Makes about 3 cups.