Cook Swiss chard in large pot of boiling salted water until just tender, about 5 minutes. Drain. Squeeze chard dry, then finely chop enough to measure 1 1/2 cups. Transfer to large bowl.
Melt butter in large nonstick skillet over medium-high heat. Add wild mushrooms, onion and 1 minced garlic clove and sauté until mushrooms are tender, about 6 minutes. Cool. Transfer to bowl with chard. Add breadcrumbs, cream, 1 tablespoon rosemary, 1 tablespoon thyme, 3/4 teaspoon salt, and 1/2 teaspoon pepper and mix to blend.
Preheat oven to 425°F. Open butterflied lamb like book and place cut side up on work surface. Sprinkle with salt and pepper. Mix veal and egg into mushroom stuffing. Spread evenly over lamb; press to adhere. Starting at narrow end, roll up lamb tightly, enclosing filling. Fasten ends with poultry skewers. Rub outside of lamb with olive oil, then with remaining 1/2 tablespoon rosemary, and 1/2 tablespoon thyme. Sprinkle with salt and pepper. Tie lamb with kitchen string at 2-inch intervals to hold shape. Using small sharp knife, cut several 1-inch-deep slits in top of lamb, spacing apart; insert 1 garlic slice into each slit. Place lamb on rack in roasting pan.
Roast lamb to desired doneness or until instant-read thermometer inserted into thickest part of lamb registers 135°F to 140°F for medium-rare, about 1 hour 25 minutes. Remove from oven. Cover with foil and let stand 15 minutes. Remove skewers and string. Cut lamb into 1/2-inch-thick slices. Place on platter.