All the peppers, onions, Mexican spices and creamy guacamole you love in fajitas … jam-crammed into one gloriously, deliciously satisfying burger recipe!
In a small bowl, combine chili powder, cumin, garlic powder, onion powder, and 1 1/2 teaspoons salt to make spice mixture, stirring to combine thoroughly.
Peel the onion and cut away the stem and root ends, then cut the onion in 4 even pieces horizontally, keeping the concentric rings nestled together in planks.
You can prepare the spiced vegetables and the burger patties earlier in the day or the night before, and store them separately, tightly wrapped, in the refrigerator, until you're ready to grill them.