Pile your panini high with roast beef, pepper jelly, and gouda.
Spread 1 tablespoon jelly evenly on each of 8 slices of bread. Spread each of 4 slices with 1/2 teaspoon mustard on top of the jelly; sprinkle Gouda evenly on the same 4 slices. Top the remaining slices evenly with the thinly sliced roast beef and the watercress sprigs. Combine the bread slices to create 4 sandwiches.
Heat a panini press or a grill pan coated lightly with olive oil cooking spray. Grill the sandwiches in batches, with the press closed, for 2–3 minutes per side. If using a grill pan, cook the sandwiches over moderately high heat with a heavy skillet on top of the sandwiches, pressing down, 2–3 minutes per side or until golden. Halve the sandwiches, and serve immediately.