Pat one 8-ounce tube crescent-roll dough into an 8-by-12-inch rectangle, pressing the seams. Brush with dijon mustard; top with 1/2 cup each thawed frozen chopped spinach (squeezed dry) and grated cheddar, and 1/4 cup chopped scallions, leaving a border. Season with salt. Starting with a short side, roll into a log; pinch the edge. Slice into 8 pinwheels. Bake at 350 degrees F on a parchment-lined baking sheet, 16 to 20 minutes.