Preheat oven to 375 degrees. Cut the tops from the two heads of garlic. Place each bulb on a square of foil large enough to wrap it, drizzle each head with 1 tablespoon olive oil, and season with salt. Wrap the heads in foil, and roast the garlic until the cloves are completely softened, about 40 minutes. Let cool.

Bring a large pot of salted water to boil for the pasta. Squeeze the pulp from the garlic cloves into the work bowl of a mini–food processor (or in a bowl, if you’re working by hand). Add the anchovies, almonds, parsley, and red pepper flakes. Process to make a paste; then, with the machine running, add the remaining 6 tablespoons olive oil to make a smooth pesto. Scrape the pesto into a serving bowl, and stir in ½ cup of the pasta-cooking water to loosen it up.

Cook the spaghetti until al dente. When the spaghetti is ready, remove it with tongs directly to the bowl with the pesto. Sprinkle with the grated cheese and toss. Season to taste with salt (the anchovies and cheese are salty, so take care), add a little pasta water if it still seems dry, toss, and serve.