Combine first 7 ingredients in a small bowl; cover and let stand at room temperature 15 minutes.
Heat oil in a small nonstick skillet over medium-high heat. Add 1 mustard seed to pan; cook until seed pops (about 90 seconds). Add remaining seeds; stir to coat with oil. Cover and cook seeds for 30 seconds or until they begin to pop, gently shaking the pan. Stir in butter and cover. Cook until 1 second passes between pops, shaking pan constantly (about 30 seconds). Remove from heat. Scrape mustard seed mixture into mango mixture; stir gently. Let stand 5 minutes.
Heat pan over medium-high heat. Coat pan with cooking spray. Sprinkle beef with 1/8 teaspoon salt and pepper. Add beef to pan; cook 3 minutes on each side or until desired degree of doneness. Let rest 5 minutes. Slice across the grain into very thin slices.
Divide beef slices evenly among baguette slices. Top each bruschetta with 1 1/2 teaspoons mango mixture. Serve immediately.