Melt the butter in a skillet over medium heat. Add the onions, and cook until tender, stirring occasionally. Add the garlic, and cook for about a minute. Whisk in the flour with a fork so no lumps form. Cook and stir with the fork until browned. Remove from heat and set aside.

Preheat the oven to 350 degrees F (175 degrees C).

Place the avocado into a blender or food processor, and pour in the milk, cream, lemon juice, salt and pepper. Puree until smooth. Pour into a heat proof bowl or casserole dish, and stir in the onion and garlic mixture.

Bake in the oven for 20 minutes, or until thickened. Stir in the tequila and cheese, and return to the oven for about 5 minutes to melt the cheese. Stir and serve as a fondue.