In a blender or food processor, add tomatoes, onion, and garlic.
Purée and set aside.
Soak the rice in warm water for 5 minutes, then rinse.
Heat up cooking oil in a large pot over medium high heat. Add the rice. Fry for 8-10 minutes until golden brown, stirring occasionally to make sure the rice doesn’t stick to the bottom of the pot.
Add in the tomato purée. Cook for 5 minutes. Give it a few stirs.
Add in the chicken stock, peas and carrots, Serrano pepper, ans salt. Give it a stir. Reduce the heat to medium. Cover and cook until all the liquid has been absorbed, 10-15 minutes.
Remove from heat, leaving the cover on. Do not stir. Let rest for 10 more minutes. After that, you can fluffwith a fork and serve.