Make the pico. In a medium bowl, add all the ingredients and toss to thoroughly combine. Cover and place in the refrigerator until ready to use.
Prep the fajitas. Brush both sides of the cauliflower steaks with oil and liberally season with spices. Place on a hot grill for 3 to 5 minutes per side or until fork tender. Slice into fajita style strips and squeeze with lime juice before serving.
While the cauliflower cooks, add a cast-iron skillet to one side of the grill. Once heated through, add a bit of oil to the skillet and then the sliced peppers and onion. Continue to toss and turn the pepper-onion mixture until the onions are translucent and the vegetables are tender and slightly charred.
Assemble: take a heated tortilla and pile with cauliflower fajita strips, a bit of grilled peppers and onions, and a generous spoonful or two of the spring green pico.