In a large pot, heat 1 Tablespoon oil over a medium heat. Add chopped onions, 1 Tablespoon (or more), minced serrano chile, and sauté about 8 minutes, stirring occasionally.
Add 4 cloves minced garlic and sauté another minute or two. Until garlic is fragrant.
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, 1/2 cup olive oil, white wine vinegar, 1 garlic clove, cumin and 1/2 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. Store leftover rice in an airtight container in the refrigerator.