1. Preheat oven to 375°.
2. Heat a large ovenproof skillet over medium-high heat. Add oil to pan; swirl to coat. Sprinkle lamb with 1/2 teaspoon salt and pepper; add lamb to pan. Sauté 10 minutes, turning to brown on all sides. Place pan in oven; bake at 375° for 15 minutes or until a thermometer inserted in thickest portion registers 130°. Remove lamb from pan; let stand 10 minutes.
3. Place pan over medium-high heat. Add onion and garlic to pan; sauté for 3 minutes, stirring occasionally. Add red wine; bring to a boil. Cook until liquid almost evaporates, scraping pan to loosen browned bits. Stir in remaining 1/4 teaspoon salt, 1 cup broth, juice, and sugar; bring to a boil. Cook until reduced to 1/2 cup (about 10 minutes), stirring occasionally. Combine remaining 1/2 cup broth and flour, stirring until smooth. Add flour mixture to pan; bring to a boil. Cook 1 minute, stirring occasionally. Strain mixture through a fine-mesh sieve; discard solids. Serve sauce with lamb.