Heat vegan butter or olive oil in a nonstick skillet over medium-high heat. Add in onions and sauté 3-5 minutes, until they start to brown slightly.
Stir in mushrooms and sprinkle with salt to draw out moisture. Sauté, stirring often, until the extra moisture from the mushrooms has cooked off and there is no extra liquid in the pan.
Add in garlic and sauté for another minute.
Sprinkle flour over the mushroom mixture and stir until there is no more dry flour. Continue to cook and stir for about a minute to cook out the raw flavor of the flour.
Pour in veggie broth to deglaze the pan, scraping up anything that's sticking to the bottom. Stir thoroughly to dissolve the flour into the broth. The mixture should be at a simmer and beginning to thicken. If it's bubbling too vigorously, reduce to the heat to medium or medium-low.
Stir in the homemade cashew cream or store-bought vegan sour cream. Taste and adjust salt and pepper to preference.
Enjoy over buttered pasta, or over rice or mashed potatoes.