• Heat oven to 350°F.
• Cut one pork tenderloin lengthwise about three-fourths of the way through to within 1/2 inch of the edge. Open tenderloin to lie flat. Cover with plastic wrap. Flatten tenderloin to 1/4-inch thickness; remove plastic wrap. Season salt and pepper. Repeat step with remaining tenderloin.
• Combine parsley and egg in a bowl. Melt 2 tablespoons butter in a large skillet over medium heat. Add onion; cook and stir for 4-5 minutes or until tender. Add Dijon mustard and lemon juice; cook and stir for 1 minute. Stir in bread crumbs; season with salt and pepper. Add to parsley mixture; toss to coat.
• Top each tenderloin with half of the fontina and parsley mixture to within 1/2 inch of edges. Roll up each tenderloin, jelly-roll style, starting with the long side. Secure with twine.
• Melt remaining butter in a large, heavy skillet over medium heat. Add one tenderloin; cook for 2-3 minutes on each side or until lightly browned. Repeat with remaining tenderloin.
• Place tenderloins on a greased 15 x 10-inch baking pan. Bake for 25-40 minutes or until a thermometer inserted into meat reads 145°F. Transfer tenderloins to a cutting board; tent with aluminum foil. Let rest for 10 minutes. Discard twine. Cut into 1/2-inch slices.

Fontina Cheese Sauce:

• Meanwhile, melt butter in the same skillet over medium-low heat. Add garlic; cook and stir for 1 minute. Stir in flour until light brown. Gradually stir in wine and lemon juice. Bring to a boil, stirring to loosen any browned bits from bottom of pan.
• Reduce heat to medium-low. Gradually add chicken broth. Bring to a boil, stirring frequently. Reduce heat to medium-low. Simmer, uncovered, for 5-6 minutes or until sauce is thickened, stirring frequently.
• Remove from the heat. Gradually stir in fontina until melted. Season with salt and pepper. Serve sauce with tenderloins.