Brown the mushrooms. Melt the butter in a large, deep sauté pan over medium-high heat. Add the sliced mushrooms and season with 1 teaspoon Diamond Crystal kosher salt and ¼ teaspoon freshly ground black pepper. Cook for 10–15 minutes, stirring occasionally, until the mushrooms release their moisture, most of the liquid evaporates, and the mushrooms are nicely browned. Adjust the heat as needed to prevent the butter from burning.
Season the mushrooms. Reduce the heat to medium. Add the garlic powder and poultry seasoning, stirring to coat the mushrooms. Cook for about 30 seconds, until fragrant.
Make the roux. Sprinkle the flour over the mushrooms and stir until completely incorporated. Continue cooking for 2–3 minutes, stirring frequently, until the flour is lightly golden and the mushrooms are coated in a thick paste with no visible dry flour.
Add the broth. Gradually whisk in the chicken broth and beef broth, adding a little at a time and stirring until smooth before adding more. Continue until all the broth is incorporated and no lumps remain.
Simmer the gravy. Stir in the Kitchen Bouquet and bring the gravy to a gentle simmer over medium heat. Reduce the heat to medium-low and cook for 5–10 minutes, stirring frequently, until the gravy reaches your desired consistency. The gravy will continue to thicken slightly as it cools.
Add pan drippings, if using. If serving with roasted chicken or turkey, skim the excess fat from the pan drippings and strain if desired. Gradually stir the drippings into the finished gravy, a few tablespoons at a time, tasting as you go. Simmer for an additional 2–3 minutes to incorporate the flavors.
Adjust the seasoning and consistency. Taste the finished gravy and season with additional salt and freshly ground black pepper as needed. If the gravy is too thick, stir in a splash of additional chicken broth until it reaches your preferred consistency.
Serve or make ahead. Serve warm, or allow the gravy to cool and refrigerate in an airtight container for up to 2 days. Reheat gently over medium-low heat, stirring frequently and adding a splash of chicken broth if needed to loosen the gravy.