To the same skillet, over medium low heat, melt the butter and add the onions. Sauté until translucent and beginning to caramelize, about 8-10 minutes. Meanwhile, prepare the sauce by adding the beef broth, Worcestershire sauce, coconut aminos, maple syrup, and cornstarch to a jar. Shake to combine (or whisk in a bowl) and set aside.
Add the mushrooms to the onions and sauté for 5 more minutes, until softened. Season with salt and pepper.
Add the garlic and cook for 1 minute, careful not to burn the garlic.
Add the sauce to the pan and scrape up all the brown bits from the bottom of the pan. Bring to a simmer.
Turn the heat to low. Return the patties to the skillet, cover, and simmer for 10 minutes or until patties are fully cooked through. Be careful not to stir much or the patties will break apart.
Taste sauce and adjust seasoning as necessary. If it needs more acidity, add more Worcestershire sauce. If it is too bitter, add a swig of maple syrup. If it needs more flavour add more salt and pepper or coconut aminos.
Serve over mashed potatoes, GF pasta noodles, or rice, with some steamed green veggies or a salad. Garnish with fresh chopped parsley or chives if desired.