Bring stock or water (pinch of salt if using water) to boiling in a 2 quart sauce pan. Add rosemary if using. Slowly whisk in the cornmeal to the boiling stock / water, whisking constantly. Cook and whisk until mixture returns to boiling. Reduce heat to very low. Cover and simmer for 15 minutes, stirring occasionally. Add Parmesan cheese and olive oil / butter.
Pour into a greased 8x4x2-inch loaf pan. Cool 1 hour. Cover and chill several hours or over night. Polenta may be kept in the refrigerator for up to 3 days before serving. To serve, remove polenta from pan and cut into 1/2-inch-thick slices.
**To bake, place slices in a single layer on a greased baking sheet. Bake, covered, in a 400 degree F oven for 10 to 12 minutes or until heated through.
**To fry, heat 2 teaspoons olive oil in a large nonstick skillet or on a griddle over medium heat. Add polenta slices and cook about 6 minutes or until golden brown, turning once during cooking time.
**To toast, lightly brush slices with olive oil and arrange on a lightly oiled baking sheet. Place under broiler, 4 to 5 inches away from the heat source, for 4 to 5 minutes per side or until golden brown and crisp.