Preheat your oven to 400°F (200°C).
In a large mixing bowl, toss the bell peppers, zucchini, and red onion with olive oil, salt, and black pepper.
Spread the vegetables out on a baking sheet in a single layer.
Roast in the preheated oven for 25-30 minutes, or until the vegetables are tender and slightly charred.
Remove the vegetables from the oven and let them cool slightly.
In a large salad bowl, combine the roasted vegetables, cherry tomatoes, and baby spinach.
Drizzle with balsamic vinegar and toss to combine.
Serve immediately or refrigerate for later.