For the pork: Stir together the garlic salt, black pepper, dried marjoram (or parsley), dried oregano and garlic powder in a small bowl, then rub it all over the tenderloins. Let them sit at room temperature while you preheat the grill. Prepare the grill for direct and indirect heat. Preheat to high (450 to 500 degrees). If you can’t grill, cook in preheated oven (using thermometer to 140-145 degrees). If grilling, close the lid and cook for about 6 minutes per side, trying to turn them only once.
Move them to the indirect-heat side of the grill. At this point, use the thermometer to begin checking their internal temperature, which should register in the 140-145 degree range (lightly pink at the center). This round of cooking should take about 8 minutes. Transfer them to a plate; cover loosely with aluminum foil and let them rest for 10 minutes before cutting.
For the ragu: Heat the oil in a medium saucepan over medium heat. Stir in the onion and cook for 3 to 5 minutes, until softened, then add the garlic and crushed red pepper flakes. Cook for about a minute, then stir in the greens (you may need to do this in two batches); cook for a few minutes until they begin to wilt.
Reduce the heat to medium-low; add the diced tomatoes, stirring to incorporate. Cook for 10 minutes, then taste and add more black pepper, as needed. (I would prepare ragu ahead of time and let it cook longer, even add some liquid to it (wine if you have it available).
While the ragu finishes cooking, cut the tenderloins into 1/4-inch thick slices, arranging them on a platter as you work. Spoon some of the warm ragu over them, sprinkle lightly with fresh tarragon and flaky sea salt, if desired, and pass the remaining ragu at the table.