In large skillet, over medium heat, cook sausage and mushrooms in butter, until sausage is brown. Add bouillon cubes and sherry and stir until bouillon is dissolved. Add zucchini and onion and cook, uncovered, until zucchini is crisp-tender.
While the sauce is cooking, bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally until cooked through but firm to the bite, about 12 minutes. Drain.