Preheat the oven to 400°F.
Heat a skillet over medium heat and add the olive oil. Add in the onions, peppers and garlic with a pinch of salt and pepper. Cook, stirring often, until the onions soften, about 5 minutes. Stir in the spinach until it wilts. Turn off the heat and let the mixture cool slightly.
Carefully slice the turkey tenderloins lengthwise. We like to slice it twice to open it up, so we have a lot of space to work with. I start at the bottom of the tenderloin (about 1/4 inch from the bottom) and slice it lengthwise, then continue to cut again and open up the turkey. You want to have a flat surface to work on. Once the tenderloins are sliced open, I take a meat tenderizer and pound them out to make them one even layer.
Add half of the onion/pepper mixture to the top of each tenderloin leaving an inch or so around the edges so you can roll it up. Sprinkle each with half of the cheddar cheese.
Tightly roll up the tenderloin and tie it together with kitchen twine if needed.
Place the tenderloins on the baking sheet and roast for 20 to 25 minutes, or until the inside is cooked and reads about 160°F. Let the turkey rest for 10 minutes before slicing.
Serve with the lime black bean salad.