Heat a skillet over medium-high heat; cook and stir pine nuts in the hot skillet until toasted and fragrant, about 5 minutes.
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, 4 to 5 minutes. Drain and return pasta to pot to keep warm.
Heat olive oil in a skillet over medium heat; saute onion and garlic until onion softens, 5 to 10 minutes. Add zucchini, lemon juice, Italian seasoning, salt, and pepper; cook and stir until zucchini is translucent, about 5 minutes.
Pour milk over zucchini mixture and simmer until zucchini is softened, 5 to 10 minutes; add pasta and toss. Transfer pasta mixture to plates and top each with Parmesan cheese, pine nuts, tomato, and black pepper.