Heat oven to 375°F; butter metal 9-by-13-inch pan. Press one dough sheet over bottom; bake 8 minutes until surface dry.
Meanwhile, brown sausage and onion in skillet over medium-high 10 minutes, crumbling meat, to 160°F. Drain fat.
Reduce to low; stir in cream cheese, mustard, and pepper until cheese melts. Spread evenly over partly baked crust.
Lay second dough sheet over filling; gently stretch to cover. Brush with beaten egg; cut six short top slits.
Bake 20 to 25 minutes until deeply golden and bottom crust firm when lifted at corner. Rest 15 minutes; cut eight squares.