In food processor, combine basil, garlic, salt, pepper, lemon zest and juice, and cashew cream. Purée until smooth.
Cook the spaghetti. Once cooked, drain, reserving a couple of tablespoons of the cooking water.
Stir the pesto cream sauce into the pasta, along with about 2 tablespoons of the reserved water, or enough to make a creamy sauce. Garnish with chopped heirloom tomatoes. Taste and season if necessary.