Preheat oven to 350 degrees F (175 degrees C). Arrange bread slices on a baking sheet. Place fava beans, olive oil, shallot, lemon juice, garlic, and cumin in a food processor. Blend until smooth. Season with salt and pepper. Combine mayonnaise, eggs, cornichons, chervil, mustard, and capers in a small food processor. Whip until gribiche sauce is thoroughly combined. Bake bread in the preheated oven until lightly toasted, 5 to 7 minutes. Top bread with hummus, smoked trout, pickled onions, and the gribiche sauce.