1. Preheat oven to 325°. Make 20 small, deep cuts in roast, and insert garlic pieces. Tie roast with kitchen string, securing at 2-inch intervals. Rub Cajun seasoning onto roast.
2. Cook roast in hot oil in a large heavy skillet over high heat 2 minutes on all sides or until browned. Remove from skillet. Reduce heat to medium. Add onions and celery to skillet; cook, stirring frequently, 5 to 8 minutes or until tender. Place onion mixture in a roasting pan; top with roast. Add broth to pan. Cover loosely with heavy-duty aluminum foil.
3. Bake at 325° for 3 1/2 to 4 hours or until a meat thermometer inserted into thickest portion registers 180° to 185°. Remove roast from pan; cover with foil, and let stand 20 minutes before slicing.
4. Pour pan drippings through a wire-mesh strainer into a measuring cup to equal 2 cups, adding additional broth or water, if necessary. Discard solids.
5. Whisk together 3 Tbsp. flour and 5 Tbsp. cold water in a medium saucepan. Whisk in pan drippings. Cook, whisking often, over medium-high heat 6 to 7 minutes or until thickened. Serve with pork. Garnish, if desired.