• Start heating 4 quarts salted water in a large stockpot over high heat. Cover cut surface of 1 avocado half and set aside. Peel and coarsely chop remaining 3 avocado halves and place in blender with 2 cups of the cream, the salt, and pepper. Blend mixture until avocado is pureed, about 2 minutes. Add remaining 2 cups cream and blend until mixture is smooth and thick, about 30 seconds.
• Heat 2 tablespoons of the oil in a large skillet over medium heat. Add the onion and garlic and sauté, stirring occasionally until it begins to brown, about 4 minutes. Reduce heat to low and add avocado-cream mixture. Simmer, stirring frequently, until volume is reduced by about half (to about 2 1/2 cups), 6 to 8 minutes. Remove from heat and stir in cheese.
• Meanwhile, add pasta to boiling water and cook until al dente, 8 to 10 minutes. Drain pasta very well; toss with sauce and transfer to large pasta bowl or shallow casserole and keep warm.
• Heat remaining 1 tablespoon oil in a medium skillet over medium-high heat. Add tomatillas and bell pepper; sauté, stirring, until hot through, 2 to 3 minutes. Add tomato and cook until tomato is hot, about 1 minute. Sprinkle over pasta. Peel and chop remaining avocado half and sprinkle over all. Serve immediately. Yield: 4 main dish servings, 6 to 8 appetizer servings (about 9 cups)