Halve and core sweet peppers. Place peppers, cut side down, on broiler-pan rack.
Broil 3 inches from heat 12 to 15 minutes, turning until skins begin to blister and blacken. Transfer peppers to a brown paper bag or glass bowl; seal tightly. Let stand for 5 minutes.
Once peppers are cool enough to handle, peel off blackened skin and discard. Slice each half into quarters.
Whisk together half-and-half, milk, flour and Parmesan in medium-size saucepan. Bring to a simmer over medium-high heat. Add salt, black pepper and spinach. Lower heat; simmer until spinach is wilted, about 3 minutes. Stir in pepper strips and cayenne; heat through, stirring, about 3 minutes.
Meanwhile, cook fettuccine with garlic in large pot of lightly salted water until al dente, firm but tender, about 10 minutes. Drain.
Transfer pasta to large platter or serving bowl. Top with roasted pepper sauce. Sprinkle with toasted nuts. Serve immediately. Makes 6 servings.