Preheat the oven to 400°F (200°C). Toss the sweet potatoes, Brussels sprouts, and squash with one tablespoon of olive oil, cinnamon, salt, and pepper. Spread them on a baking sheet and roast for 25-30 minutes until caramelized and tender, the edges slightly browned and fragrant.
While the vegetables roast, prepare the bell peppers by halving them lengthwise and removing the seeds and membranes. Drizzle the cut sides with the remaining olive oil and season with salt and pepper.
Once the vegetables are roasted and still warm, gently mash or chop them into bite-sized pieces, mixing to combine the flavors and textures. The mixture should be soft but hold together well.
Spoon the roasted vegetable mixture into each halved bell pepper, pressing gently to fill fully without overflowing. Arrange the stuffed peppers on a baking dish or sheet.
Bake the stuffed peppers in the oven for an additional 10-15 minutes, until the peppers are slightly softened and the filling is heated through, with some edges slightly browned.
Remove the peppers from the oven and let them sit for a couple of minutes to settle. Then, transfer them to plates and serve hot, enjoying the caramelized flavors and vibrant colors.
