In a large skillet or Dutch oven over medium heat, heat butter until melted. Add mushrooms and 1/2 teaspoon salt. Cook, stirring often until all liquid has evaporated and mushrooms start to brown, about 7 to 9 minutes.
Add onion and sprinkle with salt. Continue to cook, stirring frequently, until onion begins to brown and dark bits form on pan bottom, about 6 to 8 minutes.
Stir in garlic, thyme, and flour until vegetables are coated well, about 1 minute. Whisk in chicken broth. Scrape the bottom of pan to loosen browned bits, and bring to boil.
Reduce heat to medium-low and simmer until gravy thickens, about 7 to 10 minutes. Season with salt and pepper to taste (I like 1/4 teaspoon salt and 1/8 teaspoon pepper).
Notes
Mushrooms: Choose any combination of mushrooms such as white button mushrooms, cremini mushrooms, shiitake mushrooms, or baby portobello mushrooms, aiming for 1 pound total.
Flour: Substitute cornstarch for making an easy gravy that’s also gluten-free.
Yield: This recipe makes 4 generous servings. Estimate 1/4 to 1/2 cup per person.
Storage: Store leftovers in an airtight container in the refrigerator for 3 to 4 days. Or, freeze for 4 to 6 months.