In a large pot, heat 1 Tablespoon oil over a medium heat. Add onions, 1 Tablespoon (or more), minced serrano chile, and sauté about 8 minutes. Add 4 cloves minced garlic and sauté another minute or two.
Add rice, broth and salt, stir and bring to a boil. Reduce heat to low, cover and cook until rice is tender and broth is absorbed, about 15-20 minutes.
While rice is cooking, in a food processor or blender, combine cilantro, ½ cup olive oil, white wine vinegar, 1 garlic clove, cumin and ½ teaspoon (or more) minced seeded serrano chile. Puree until smooth. Set aside.
Transfer rice to a bowl and cool, stirring occasionally, about 45 minutes. Toss with cilantro dressing and adjust seasoning salt and black pepper if needed.
Serve warm or chilled.