1. Heat a small saucepan over medium-high heat. Coat pan with cooking spray. Add garlic; sauté 30 seconds or until garlic becomes fragrant, stirring constantly. Add 1/2 cup water; bring to a simmer. Add demi-glace, stirring with a whisk until combined. Combine remaining 1/4 cup water and mustard, stirring with a whisk until mustard dissolves. Stir mustard mixture, vinegar, and 1/4 teaspoon pepper into glace mixture; bring to a boil. Reduce heat, and simmer 3 minutes; strain through a fine sieve over a bowl. Discard solids; keep glace warm.
2. Heat a large heavy skillet over medium-high heat. Sprinkle remaining 1/4 teaspoon pepper and salt evenly over fish. Add fish to pan; cook 2 1/2 minutes. Reduce heat to medium; turn fish, and cook 7 minutes or until desired degree of doneness. Sprinkle with green onions, and serve with glace.