Fill a large pot with water and salt generously (about 3-4 quarts water, about 2 tablespoons salt). Bring to a boil, then cook the pasta according to the package instructions, adding the cauliflower in during the last 2 minutes of cooking time.
Reserve about 1 cup pasta water, then drain (it’s best to use a fine mesh sieve if you have one that’s big enough so the cauliflower doesn’t fall through the holes of a colander). Transfer back to the pot.
In a high-speed blender, add almonds, parmesan, lemon, salt, pepper, and garlic and blend until everything is finely chopped, scraping down the sides as needed. Scrape the sides down one final time and, with the motor running, slowly pour in the olive oil. Blend until combined.
Add the pesto to the pot along with ½ cup pasta water and stir constantly until the pasta absorbs the water and the sauce thickens, about 1 minute. Add more pasta water if it’s looking too dry.
Serve topped with chopped herbs and black pepper.