Heat olive oil in a large Dutch oven over medium-high heat; cook and stir onion until tender, 4 to 5 minutes. Add sofrito, garlic, cumin, bay leaf, salt, and pepper; cook and stir until fragrant, 2 to 3 minutes. Add chicken stock and bring to a boil.
Stir plantain into mixture and cook until tender, 10 to 15 minutes. Remove bay leaf and blend mixture with an immersion blender until soup is smooth and thick.