1. Cut each celery stalk crosswise into about 3-inch-long pieces; cut each piece into match-stick-thin strips. Set aside.
2. With knife held in slanting position, almost parallel to the cutting surface, cut tenderloin crosswise into 1/4 inch-thick slices. With meat mallet or dull edge of French knife, pound each tenderloin slice to about 1/8 inch thickness. On waxed paper, mix flour, sage, pepper, and 1/4 teaspoon salt. Coat tenderloin pieces with flour mixture.
3. In 12-inch skillet over medium heat, in 2 tablespoons hot salad oil, cook celery, garlic, and 1/2 teaspoon salt, stirring occasionally, until celery is tender. Discard garlic. Remove celery to bowl; keep warm.
4. In same skillet over medium-high heat, heat 2 tablespoons butter or margarine (1/4 stick) and 2 more tablespoons salad oil until hot. Add tenderloin pieces, a few at a time, and cook until lightly browned on both sides, removing pieces to plate as they brown and adding more butter or margarine if necessary.
5. Thinly slice half of lemon; set aside. Return tenderloin pieces to skillet, pushing them to one side; arrange celery on other side of skillet; heat through. Squeeze juice from remaining lemon half over tenderloin pieces and celery. Garnish with lemon slices.