In a deep skillet or dutch oven, melt the butter over medium heat and then add the sliced onions. Stir occasionally while doing the next steps.

Start water heating for your pasta. Salt the water enough that it tastes pleasant on its own.

Meanwhile, slice or chop your mushrooms. I use an egg-slicer to make it fast but if I'm without one, I quarter the mushrooms unless they're huge. Add them to the onions as you go.

Cook the onions and mushrooms until they've released their liquid and it's mostly evaporated, about 10 minutes. Stir frequently at this point to keep them from burning.

Start cooking your pasta according to package directions.

Add the flour to the onion-mushroom mix and stir until it's well-mixed. Cook for one minute, then add the milk. Cook, stirring frequently, until the mixture thickens. Reduce the heat to low.

Add the Greek yogurt or sour cream and stir continuously until the mixture starts to bubble just a bit. Remove from heat and then season to taste with salt and pepper.

Drain your pasta and then serve the mushroom mix on top of individual servings of pasta. Garnish with parmesan cheese and parsley.