Cooking the onions: Heat half the oil over medium-low heat. Add onion and bell pepper and cook stirring until onion becomes translucentPour in vinegar and cook until the onion becomes pink. Remove from heat and cool to room temperature.
Toasting the bread: Slice the bread. Brush the bread with the remaining olive oil. Heat a cooktop grill and toast on both sides, pressing it to flatten it.Cool to room temperature.
Cutting the avocado: Peel and remove the pit. Cut into halves and slice very thinly.
Assembling: Place the avocado slices onto the bread toastsTop the bread with the onion mix. . Season with salt and pepper to taste. Sprinkle with cilantro.
Serving and storing: Serve right away. If you wish to make it beforehand, make the onion relish and toast bread and keep them at room temperature. Cut avocado and assemble right before serving.