1) Heat the butter and olive oil in a large saucepan. Add the garlic, chilli flakes, rosemary, salt, and pepper and saute for 1 minute. Add the chicken stock, half-and-half, and milk and bring to a boil. Remove from the heat and slowly sprinkle the cornflour into the hot milk while stirring constantly with a whisk. Cook over low heat, stirring constantly, for a few minutes, until thickened and bubbly. Off the heat, stir in the Parmesan.
2) Pour into a 23 by 33 by 5cm pan, smooth the top, and refrigerate until firm and cold.
3) Cut the chilled polenta into 12 squares, as you would with brownies. Lift each one out with a spatula and cut diagonally into triangles. Dust each triangle lightly in flour. Heat 1 tbsp olive oil and 1 tbsp butter in a large saute pan and cook the triangles in batches over medium heat for 3 to 5 minutes, turning once, until browned on the outside and heated inside. Add more butter and oil, as needed.
4) Serve immediately.