Frozen puff pastry instead of a from-scratch crust speeds up this version of the classic comfort food.
Line a baking sheet with parchment and preheat oven to package directions (375˚F). On a lightly floured surface, roll out puff pastry sheet and cut 4 squares about 5x5-inch or 6x6-inch or 4 circles about the same size in diameter. Cut a small cross or a few air vents at center of each square, or use a small cutter to cut out a shape like a heart or small circle. Arrange the dough squares on parchment and brush with egg and bake to evenly golden and puffed.
Slice into the chicken breasts horizontally across the breast to butterfly them open, gently spread with a mallet and season both sides with salt and pepper.
Cook 2 large chicken cutlets at a time in large skillet over medium-high heat about 6-7 minutes, in EVOO, 1 tablespoon or 1 turn of the pan for each set of cutlets, place on small tray and keep warm covered with foil or in a low temp oven.
To the skillet, add butter, and when it foams, add vegetables, garlic, salt, pepper and thyme. Add flour and stir, then whisk in wine and stock and let it bubble and thicken a bit. Add peas and heat them through, 2 to 3 minutes, then remove from heat and add mustard and tarragon to sauce.
Arrange chicken on plates, top with gravy and vegetables, and top sauce with puff pastry.