Bring a medium saucepan of salted water to the boil for the spaghetti. Finely chop the onion. Thinly slice the capsicum. Crush or finely chop the garlic. Quarter the zucchini lengthwise and thickly slice.
Step 2 — Cook onion
Heat 1 tbs olive oil in a medium deep frypan over medium-high heat. Cook the onion, stirring regularly, for 2 mins or until beginning to soften.
Step 3 — Simmer pasta sauce
Add the capsicum and garlic to the onion and cook, stirring, for 2 mins or until softened. Add the tomatoes, zucchini, 1 tsp dried oregano*, 1 tsp sugar and 1 tsp balsamic vinegar, season with salt and pepper and bring to a simmer. Reduce the heat to medium-low and cook, covered, for 10 mins or until the vegetables are tender. Remove from the heat.
Step 4 — Cook spaghetti
Meanwhile, add three-quarters of the spaghetti* to the pan of boiling water (if you're tempted to throw in all the spaghetti, just note the dish won't be as saucy). Cook, stirring occasionally, for 12 mins or until al dente. Drain.
Step 5 — Wilt spinach
Add the spinach to the pasta sauce and stand, covered, for 2 mins or until the spinach is wilted. Taste, then season with salt and pepper.
Step 6 — Get ready to serve
Finely chop the parsley leaves, discarding the stems. Stir the parsley into the pasta sauce. Coarsely grate the cheese. Divide the spaghetti and pasta sauce among bowls. Scatter with the cheese to serve.