Preheat your grill or oven to 400°F (200°C).
Peel the Tennessee onions and cut them into thick slices.
In a large bowl, combine olive oil, salt, pepper, garlic powder, and smoked paprika.
Add the onion slices, bell pepper, zucchini, and yellow squash to the bowl.
Toss everything until the vegetables are well coated with the seasoning mixture.
Place the veggies on a grill basket or a baking sheet lined with parchment paper.
Grill or roast the vegetables for about 25-30 minutes, turning occasionally.
Check for tenderness; the onions should be caramelized and soft.
Once cooked, remove from heat and let them cool for a few minutes.
Garnish with fresh herbs if desired, and serve warm.