Place onion slices and garlic on a rimmed half sheet and toss brush with remaining 1 1/2 Tbsp olive oil on all sides, sprinkle with salt and pepper.
Place baking sheet with tomatoes and half sheet with onions in oven side by side and roast onions 30 - 35 minutes until edges are golden, and tomatoes 40 - 45 minutes until golden.
Peel away any burnt papery layers of onions if there are any. Pull peels from tomatoes.
Blend soup with an immersion blender or in small batches in a blender (only fill blender half full and remove center insert and cover with a kitchen towel). Serve warm topped with grilled cheese croutons if desired.