Preheat oven to 350. Toast cumin, coriander, and fennel seeds on a sheet pan in the oven for 4-5 minutes. They will be fragrant. Let cool slightly.
If you have a gas stove, turn the heat on to medium-high and place jalapeños on to the grate. Turn often until they are completely blackened. Turn off flame and let cool.
Once jalapeños are cooled, use a paper towel to wipe off the outer layer. Cut off the stem and remove seeds. Roughly chop the jalapeno.