Heat broiler. Heat broiler to high and arrange an oven rack at the top of the oven.

Char vegetables. Arrange vegetables on a baking sheet and place under the broiler. Check every 2-3 minutes, turning the vegetables so they get charred on all sides and removing the ones that have blackened quicker. You want the skin of the vegetables to be blistered and black, it should take about 20 minutes total.

Blend. Remove the stems of the serrano chiles and the skin of the garlic. Transfer everything to a blender and add salt. Blend, starting on low then increasing the speed to high until very smooth.

Taste. Taste salsa with a chip. Add more salt if needed or an extra chile if you'd like it more spicy.