Preheat the oven to 350 degrees.
Make the cheese sauce: In medium saucepan over medium heat, melt butter. Add flour and whisk for 1 minute. Start adding warm milk in a thin stream, while constantly whisking so no lumps form. When all milk is added, continue stirring as sauce starts to thicken, this should take 3-4 minutes. When sauce is thick, add cheeses a bit at a time, whisking well after each addition making sure sauce remains smooth. Add salt and pepper to taste. Remove from heat and reserve.
Make the filling: In medium frying pan, heat 2 tablespoons vegetable oil over medium heat. Cook leeks until golden brown, this should take 10-15 minutes. Add 1 more Tbsp oil and add poblano strips and minced garlic to pan. Cook for another 4 minutes. Remove from pan.
Assemble: In a buttered medium casserole mold, start layering chayotes and poblano leek mixture. Add cheese sauce in between layers. Continue in this fashion until all vegetables and cheese sauce are used up, finishing with cheese sauce on top.
Cover with parchment and then tinfoil and bake in pre-heated oven for 20 minutes. Remove parchment and tinfoil and bake another 20 minutes or until golden and bubbly. Top with amaranth and bake another 3 minutes. Serve warm.