Start by making the polenta. Shuck corn and slice kernels off cobs into a large saucepan. Milk the cobs by scraping with the back of your knife to extract remaining corn juice, this step adds additional creaminess, don't skip it. Discard cobs.
Add just enough water to the saucepan to cover the kernels. Add 1/2 teaspoon salt, stir, and cover. Bring to a gentle boil over high heat, then reduce to low and simmer covered for 8 minutes. Strain corn, discarding cooking liquid. Set pot aside.
Process corn and pepper in food processor or blender until almost smooth (adjust to desired texture). Return corn to pot. Add remaining salt, butter, and parsley. Cook over low heat, stirring constantly, for 3 minutes. Season with additional salt to taste.
Make the Ragu. Heat olive oil in a large skillet over medium heat. Add onion and cook 5-6 minutes until lightly browned. Add mushrooms and thyme. Cook, stirring occasionally, until mushrooms are golden, about 5-6 minutes.
Add cherry tomatoes, salt, and pepper. Cook until tomatoes burst, about 6-7 minutes. Stir in marinara sauce to coat vegetables. Add wine and bring to a simmer. Cook until wine is mostly reduced, about 5-6 minutes.
Serve ragu on top of polenta and garnish with parsley.