First, prepare the escabeche by heating a large sized saucepan to stovetop medium heat. Pour in a little bit of olive oil for sautéing. Toss in the half moon sliced red onion and minced garlic after the pan is ready. Sauté the mixture for 3-5 minutes while continuously stirring.
Slowly pour in the olive oil and apple cider vinegar. Add in the spanish olives, bay leaves, 1 tsp salt, and the black peppercorns. Stir the mixture around and reduce the heat to simmer. Set aside for 50 minutes.
Heat a large pot to medium-high heat. Cut the tips of the unripe bananas and add to your large pot with the skin still on. Add water to your large pot slightly covering the bananas. Add the other 1 tsp of salt into the water. Remove bananas from the water when the skin begins to split. Then, peel the skin off of the bananas when they cool off.
Finally, cut the bananas into 1-inch slices and place into a large food storage (preferably glass). Pour escabeche over the sliced bananas and mix in the avocados. Store in the refrigerator for at least 12 hours before serving.