Heat oil in medium-size saucepan over medium heat. Add onion and bell pepper, cover, and cook 10 to 15 minutes, or until vegetables are soft, stirring occasionally. Stir in mango chutney and tomato. Cover, and simmer 5 minutes more. Keep warm.

Preheat oven to broil, and place oven rack in highest position. Coat baking sheet with nonstick cooking spray. Halve each tofu block crosswise to make pieces the size and thickness of sandwich bread. Cut each tofu piece into 4 triangles. Brush each tofu triangle on both sides with mango mixture (it's OK if some bell pepper and onion bits stick to tofu); season with salt and pepper, if desired; and place on prepared baking sheet. Broil 4 to 5 minutes. Flip triangles, and brush with more mango mixture. Broil 4 to 5 minutes more, or until browned and crispy.

Meanwhile, bring 1 cup water to a boil in large pot. Add mustard greens, cover, and steam 5 to 7 minutes or until greens are crisp-tender, turning occasionally with tongs to make sure greens cook evenly.

Divide mustard greens among serving plates. Top each serving with 2 tofu triangles, and drizzle with1/4 cup mango sauce.