Bring a large pot of salted water to a boil and cook the pasta according to package instructions until al dente. While the pasta cooks, toast the almonds in a dry skillet over medium heat until golden and fragrant, about 3-5 minutes. Remove from heat and let cool slightly. In a food processor, combine the toasted almonds, fresh basil leaves, garlic, grated Parmesan cheese, and lemon juice. Pulse until coarsely chopped. With the processor running, slowly add olive oil until the mixture is smooth and emulsified. If the pesto is too thick, add a little water or reserved pasta cooking water to reach desired consistency. Season the pesto with salt and freshly ground black pepper to taste. Drain the cooked pasta, reserving about 1/4 cup of the pasta cooking water. In a large bowl, toss the pasta with the almond pesto, adding reserved pasta water as needed to help the sauce coat the pasta evenly. Serve immediately, garnished with extra Parmesan cheese and a few toasted almond slices if desired.