Half-fill a small pot with water and add diced carrots. Bring to a lively simmer and cook for 3 minutes. Add ½ cup peas; cook until the peas are heated through and the carrots are just tender, about 2 minutes more. Drain; set aside to cool for 5 minutes.
Meanwhile, combine ¼ cup mayonnaise and 2 tablespoons lemon juice in a medium bowl. Add 2 cans tuna including the oil from the cans, ½ cup cilantro and 4 teaspoons serrano (or jalapeño); use a fork to combine, stirring and breaking up larger chunks of tuna as needed.
Add the carrots and peas and ⅛ teaspoon salt to the tuna mixture; stir until well combined.
