Heat oil in a large skillet over medium-high heat. Add onion and cook until soft and translucent, about 5 min. Add meatless grounds and break up the clumps with a spoon until crumbly. Cook until browned and heated through, about 5 minutes. Reduce heat to low. Add ketchup, brown sugar, apple cider vinegar, ground mustard, dill, and pepper. Mix well, cover, and let simmer until heated through, 3 to 4 min. Adjust seasonings if necessary. Serve on hamburger buns with desired toppings and sides. Yields four sandwiches.