Make the filling: In large sauté pan heat 1 tablespoon oil. Cook onions over medium high heat for 2 minutes or until softened. Add mushrooms, epazote, and salt and sauté until mushrooms release their liquid and are browned and crispy, about 5 minutes. Remove to a plate and cool completely.
Make the sauce: in blender container, combine tomatoes, onion, garlic, chipotle chiles, epazote, and kosher salt. Blend until completely smooth. Set aside.
Make pork: Using a large knife, butterfly pork tenderloin. Cover with plastic wrap and pound with mallet until it’s just a bit over ¼” thick. Mound cooled sautéed mushroom mixture down the center and roll up down the long end, jelly-roll style. Secure seams with toothpicks.
In same sauté pan, heat remaining oil. Brown pork tenderloin on all sides over medium high heat, about 5 minutes. Make sure it has a nice golden brown color. Pour sauce over tenderloin and turn heat down to simmer. Cover pan and cook for 25-30 minutes or until tenderloin is cooked through.
To serve: remove tenderloin to cutting board, remove toothpicks, and let rest 5 minutes. Keep sauce warm. Slice tenderloin into 1” pieces and arrange on a platter. Spoon sauce all around tenderloin. Garnish with fresh epazote leaves.