Line a shallow rectangular casserole dish of approximately 21x32 cm/ 8x13 inches with baking paper.
Make polenta: Bring the water to a soft simmer, add the salt, stir well to dissolve it, and then slowly add the polenta while stirring or whisking continuously. Cover and let cook on very low heat, stirring with a wooden spoon from time to time, according to the packet's instructions.
Cooking time: My polenta usually needs about 20 to 30 minutes, but there are quick-cooking sorts that only need 10 minutes and other kinds that require between 40 and 50 minutes. So make sure you follow the packet's instructions.
Let cool: When the polenta is ready, pour it into the prepared dish. Level nicely and leave it to cool.
When the polenta is cool, preheat the oven to 200 degrees Celsius/ 400 degrees Fahrenheit.
Clean and slice the mushrooms. Halve the sage leaves and cut them into thin strips.
Cook mushrooms: Melt the butter in a large pan and fry the mushrooms and the sage for about 3-4 minutes, until golden. Season well with salt and pepper.
Bake: Pour the contents of the pan, juices included, over the cold polenta. Spread evenly and top with the grated cheese. Dot with the extra teaspoon butter and bake for about 20 minutes or until the cheese is melted and golden.
Mix: Grate the garlic cloves into a small bowl, add the mustard and some salt and stir well.
Slowly add the vegetable oil while stirring until the mixture emulsifies slightly.
Add the Greek yogurt and stir well. Adjust the taste with salt and pepper.
Serve the polenta casserole immediately with the yogurt sauce and a green salad.