Prepare Cilantro Sauce: Place a small skillet over medium-high heat until hot; add cumin seeds, and cook, stirring constantly, 1 to 2 minutes or until toasted. Cool 10 minutes. Process cilantro, next 4 ingredients, 2 Tbsp. water, and toasted cumin seeds in a blender until smooth, stopping to scrape down sides as needed. Cover and chill until ready to serve.
Prepare Relish: Sauté onion in 1 tsp. hot olive oil in a small skillet over medium-high heat 6 to 8 minutes or until onion is tender. Transfer to a medium bowl, and stir in mango, next 3 ingredients, and salt and pepper to taste.
Prepare Beef Tenderloin: Rub steaks with 1 Tbsp. olive oil. Sprinkle with 1 tsp. salt and next 2 ingredients. Place a grill pan over medium-high heat until hot; cook steaks 8 minutes on each side or to desired degree of doneness. Let stand 5 minutes. Thinly slice steak.
Assemble: Top flat sides of Herbed Cornbread Crostini with relish and steak; drizzle with Cilantro Sauce.