Using a medium sauce pot, heat the avocado to medium heat. Sautee the onion and garlic for 4-5 minutes.

Stir in the remainder of the ingredients (reserving the horseradish) and bring to a simmer. Simmer for 15 minutes until the sauce thickens. Stir in the horseradish, little by little, and adjust the spice to taste.

Remove from heat and allow to cool. Blend with an immersion blender or high-speed blender before using.

Preheat the oven to 375 F and prepare a large, 9x13 casserole dish

Slice the ends off the zucchini and use a mandolin slicer or a peeler to slice the zucchini into long, thin, 1/8" inch strips. Lightly salt and allow to sit for 5 minutes. Pat them down with a paper towel or clean dish towel to remove excess water. Set them aside.

Line the bottom of the casserole dish with about a third of the enchilada sauce. Layer on the zucchini slices vertically (you'll have about two rows of 5-6 slices). Add about 1/3 cup of shredded chicken and layer on a few spoonfuls of enchilada sauce. Repeat this process for about 4 layers. Finish the final layer as just zucchini, and top with the remainder of the enchilada sauce.

Cover the casserole dish and bake for 20 minutes covered. Remove the cover and bake for another 5 minutes.

Top the enchilada casserole with the remainder of the ingredients, saving the lime slices for serving. To serve, use a spatula to carefully cut a slice of the enchiladas. Serve with limes and additional toppings to taste.