Microwave onions & mushrooms on high 3 minutes. Saute onions & mushrooms 3 - 4 minutes. Mix mustard & maple syrup and set aside. Combine half the onion/mushroom mix & half the mustard/syrup mix with the ground beef. Salt & pepper to taste. Form into 4 hamburgers, thicker on the edges. Saute over medium-high heat for 2 minutes, turn and saute 3 minutes. Place burgers on the bun heel, top with remaining onion/mushroom mix and mustard/syrup mix.