Roast the garlic. Preheat the oven to 400°F (200°C). Slice the tops from the garlic heads to expose the cloves, drizzle lightly with olive oil, wrap in foil, and roast for about 40–50 minutes, or until very soft. Cool slightly, squeeze out the cloves, and mash. Set aside.

Roast the red peppers. Set the oven to broil. Arrange the pepper quarters skin-side up on a foil-lined baking sheet. Broil for approximately 10–15 minutes, watching carefully, until the skins are well blistered and darkened.

Steam and peel the peppers. Transfer the hot peppers to a covered heat-safe bowl and let them steam for about 10 minutes. Peel away the loosened skins and roughly chop the peppers.

Cook the fettuccine. Bring a large pot of salted water to a boil. Cook the fettuccine according to package directions until al dente. Reserve about 1 cup of pasta water, then drain.

Sauté the onion. Heat the olive oil in a large pot or deep skillet over medium heat. Add the diced onion and cook for about 5 minutes, or until softened and translucent.

Blend the sauce. Add the roasted peppers, roasted garlic, and cooked onion to a blender or food processor. Blend until smooth.

Build the Alfredo sauce. Melt the butter in the same pot over medium-low heat. Pour in the red pepper puree, followed by the broth, heavy cream, and milk. Season lightly with salt and black pepper. Stir until smooth and gently heated through.

Add the cheese. Reduce the heat to low or remove the pan briefly from the heat. Stir in the freshly grated Parmesan until melted and creamy. Add the parsley.

Combine with pasta. Add the cooked fettuccine in batches and gently toss until every strand is coated in the Roasted Red Pepper Fettuccine Alfredo sauce. If the sauce is too thick, loosen it with a small amount of reserved pasta water.

Serve. Divide between bowls and finish with extra grated Parmesan, fresh parsley, and black pepper. Serve immediately.