1. Heat a large saucepan over medium-high heat. Coat pan with cooking spray. Add sausage; cook 2 minutes or until sausage begins to brown, stirring to crumble. Add onion, celery, and garlic; sauté 8 minutes or until vegetables are tender.
2. Add 1 1/2 cups water and milk to sausage mixture in pan, and bring to a boil. Gradually add polenta, stirring constantly with a whisk. Stir in salt. Cook for 3 minutes or until thick, stirring constantly. Spoon polenta into an 11 x 7inch glass or ceramic baking dish coated with cooking spray. Cool to room temperature; cover and refrigerate at least 4 hours.
3. Cut the chilled polenta into 8 squares, and cut each polenta square diagonally into triangles. Heat a large nonstick skillet over medium-high heat. Add 1 tablespoon olive oil; swirl to coat. Place 8 triangles in pan; cook for 2 minutes on each side or until browned. Repeat procedure with remaining olive oil and polenta triangles.