Preheat the oven to 400 degrees. Using a tofu press or a kitchen towel, press out as much moisture as you can from the tofu. The best way to do this with a kitchen towel is to wrap the tofu in the towel. Then place a heavy skillet and some cans or other weight inside on top. Let sit on the counter for 10-15 minutes. Slice the tofu into about 24 thin strips. You can also cube the tofu if you prefer smaller pieces.
Toss the peppers and onions with half of the oil and fajita seasoning. Spread out on the sheet pan on one side. Then carefully toss or brush the tofu with the remaining oil and coat with spices on both sides. Lay out flat on a baking sheet. If you do not have enough room on the sheet pan for everything, use two baking sheets so that everything can crisp up and brown.