Thinly slice the capsicum. Halve the zucchini lengthwise, then thinly slice. Crush or finely chop the garlic. Crumble 1 stock cube* into a heatproof jug, add 125ml (½ cup) boiling water and stir to dissolve.
Step 2 — Prep chicken
Bring a medium saucepan of salted water to the boil for the spaghetti. Put the chicken flat on a board, put your hand on top and halve horizontally, then thinly slice into strips. Season with salt and pepper.
Step 3 — Start sauce
Heat 2 tsp olive oil in a large deep frypan over high heat. Cook the chicken and 1 tsp Italian seasoning*, stirring, for 2-3 mins until golden and cooked through. Remove from the pan. Reduce the heat to medium-high. Add the capsicum and cook, stirring, for 3 mins or until softened. Add the zucchini and garlic and cook, stirring, for a further 1 min.
Step 4 — Cook sauce
Add the tomatoes, stock, ½ tsp sugar and ½ tsp balsamic vinegar to the vegetable mixture and bring to the boil. Reduce the heat to medium and cook, stirring occasionally, for 8-10 mins until the sauce is thickened. Taste, then season with salt and pepper.
Step 5 — 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 and return to the pan.
Step 6 — Get ready to serve
Add the chicken, olives and sauce to the spaghetti and toss well to combine. Divide the spaghetti among bowls and scatter over the chilli flakes, if desired, to serve.