Trim excess fat and thin membrane from leg of lamb; place into roasting pan. Cut slits in the meat and insert garlic slivers into slits. Mix olive oil, lemon juice, and rosemary in a small bowl; brush mixture over lamb. Lay bacon slices onto the meat. Let roast stand at room temperature for 30 minutes.
Preheat oven to 325 degrees F (165 degrees C).
Roast lamb in the preheated oven until an instant-read meat thermometer inserted into the thickest part of the leg, not touching bone, reads 145 degrees F (65 degrees C) for medium-rare or 160 degrees F (70 degrees C) for medium, 1 1/2 to 1 3/4 hours. Transfer lamb to a serving platter, cover loosely with aluminum foil, and let stand while you make the sauce.
Skim and discard excess grease from pan drippings left in roasting pan; place roasting pan over medium heat. Cook and stir onion in drippings until translucent, about 5 minutes; pour in wine. Stir wine and pan drippings, scraping up and dissolving brown bits of food stuck on the bottom of the pan. Pour in stock, bring sauce to a boil, and reduce heat to low. Stir tomato paste into sauce, bring to a simmer, and cook until sauce has reduced to about 1 cup. Season with salt and black pepper. For a smoother sauce, strain before serving.