Make the onion-mushroom gravy: Reduce heat to medium. Add chopped yellow onion and sliced mushrooms to the skillet and cook, stirring occasionally, until softened and lightly browned, about 3-5 minutes. Gradually whisk in beef broth, ensuring there are no lumps. In a small bowl, whisk together flour and cornstarch. Slowly add flour mixture to the skillet, whisking constantly until the gravy thickens. Reduce heat to low and simmer for 1 minute, stirring occasionally. Remove from heat and let sit for 1-2 minutes; the gravy will continue to thicken as it cools. Season to taste with salt and pepper. (Cook time: 8-10 minutes)