Heat a 10-inch cast-iron skillet over medium-high heat. Add 1 teaspoon canola oil to pan, and swirl to coat. Remove the casings from chorizo. Add chorizo to pan; sauté for 3 minutes, stirring to crumble. Add garlic, and sauté for 30 seconds. Remove sausage mixture from pan. Combine the sausage mixture and enchilada sauce.
Add remaining 3 teaspoons oil to pan; swirl to coat. Combine grated onion, salt, pepper, potato, and egg; toss. Add potato mixture to pan, pressing gently; cook for 10 minutes (do not stir). Spread enchilada sauce mixture evenly over potato mixture; sprinkle evenly with cheese. Bake at 450° for 20 minutes. Remove from oven; let stand for 10 minutes. Cut into 6 wedges.
4. Combine avocado and juice; toss gently. Stir in cilantro and remaining ingredients. Serve cilantro mixture with casserole.