Heat oil in a saucepan over medium heat. Chop onion and mince garlic; add onion to hot oil. Sauté 5-6 minutes until translucent, then add garlic and cook for an additional minute.
Chop tomatoes and add to pan, along with broth and cilantro. Reduce heat and simmer for 15 minutes.
Puree soup with an immersion blender (or food processor; take care when blending hot liquids!). Add pepper, salt, cumin, and chili powder; stir to combine and continue cooking until heated through.
Top with crumbled tortillas and diced avocado; serve watermelon on the side.