Preheat broiler to high. Cut peppers in half and remove seeds and veins. Leave stems intact. Place peppers, skin side up, on a baking sheet covered in parchment paper. Broil for 10 minutes
Meanwhile, combine quinoa, broth and tomatoes in a medium pot. Bring to a boil, then reduce heat to low and simmer for 10 minutes, until liquid is absorbed. Add spinach and stir to combine. Fill pepper halves with this mixture
Place pesto ingredients in a food processor and pulse until finely chopped. Add 2-3 tablespoons of water as needed to thin out, if desired. Top stuffed peppers with pesto or serve on the side.