Heat broiler with rack 4 inches from heat source. Place onion slices and pepper, skin side up, on a broiler pan. Broil, turning onion over halfway through cooking time, until peppers are blistered and onion is lightly browned, 12 to 15 minutes. When cool enough to handle, peel skin off pepper. Cut pepper into thick strips.
In a large skillet, heat olive oil over medium-high heat. Saute tofu until golden brown and crisp, about 2 minutes on each side.
In a small bowl, combine peanut butter, honey, 1/4 teaspoon sesame oil, and ginger. Toss reserved vegetables with remaining 1/4 teaspoon sesame oil. Spread peanut-butter mixture on bottom side of bread. Top with tofu and vegetables. Place other piece of bread on top. Cut in half to make two sandwiches. Serve with radish sprouts on the side, if desired.
Wrap tightly in foil and refrigerate for up to 2 days.