Cook bacon in a large skillet over medium-high heat, turning once, until crisp, about 7 minutes. Remove to paper towels to drain.

While bacon is cooking, slice onion thinly and separate into rings. Cook onion in bacon drippings 1 to 2 minutes or until softened.

Spread chicken evenly on both sides with mustard. Push onion rings to edge of skillet and add chicken. Cook until chicken is cooked through, 4 to 5 minutes per side.

While chicken is cooking, seed and dice tomato and set aside. Remove chicken and onion with a slotted spatula and place over toast. Top with bacon and tomato and sprinkle with pepper.