In a food processor, combine lemon juice, garlic, mustard, salt, pepper, and 1 tbsp of water, and purée until fairly smooth, scraping down sides of bowl as needed. If difficult to puree, add a touch more water (keep in mind that some pasta water will be added when tossing through pasta). Add cashews and basil and purée until fairly smooth (may leave some texture). Cook pasta according to package directions. When almost done, remove about 1/2 cup of pasta water and reserve. Drain pasta (do not rinse) and toss with pesto, using as much pesto as desired (see note). If pasta seems too dry, add some pasta water, 1 tbsp at a time, until moistened to preference. Season with additional salt and pepper if desired, and serve.