Preheat oven to 425°. Combine mustard, 2 tablespoons olive oil, and 1 tablespoon lemon juice in a shallow bowl; add salmon, turning to coat. Chill 30 minutes.
Meanwhile, arrange bread cubes in a single layer on a baking sheet. Bake 8 minutes or until golden and crisp. Set aside to cool.
Remove salmon from marinade; sprinkle with salt and pepper. Cook salmon on a lightly greased grill pan over medium heat 5 minutes on each side or until done. Transfer to a cutting board, and let cool 5 minutes. Cut into pieces or break into chunks.
Whisk together salsa, remaining 2 tablespoons olive oil, and remaining 1 tablespoon lemon juice. Season to taste with additional salt, if necessary. Place mixed baby greens and croutons in a large bowl; add salsa mixture, tossing well to coat. Divide among 4 plates, and top evenly with salmon.