How to Make It
To prepare ragù, combine 1 cup boiling water and porcinis in a bowl; cover and let stand 15 minutes. Drain porcinis through a sieve over a bowl, reserving soaking liquid. Discard porcini stems; finely chop porcini caps.
Combine 1 cup boiling water and sun-dried tomatoes in a bowl. Cover and let stand 15 minutes. Drain and finely chop tomatoes.
Heat oil in a Dutch oven over medium heat. Add onion and garlic; cover and cook 10 minutes, stirring occasionally. Add creminis, rosemary, salt, and red pepper. Cover and cook 5 minutes. Uncover and cook 5 minutes or until liquid almost evaporates.
Add porcini caps, reserved soaking liquid, chopped tomatoes, wine, and parsley; bring to a boil. Reduce heat; simmer 12 minutes.
To prepare polenta, place cornmeal and salt in a large saucepan. Gradually add 1 1/2 cups water and milk, stirring constantly with a whisk; bring to a boil. Reduce heat to medium; cook 5 minutes, stirring frequently.
Remove from heat; stir in Gruyère. Serve with ragù; sprinkle with Parmesan.