In a medium saucepan over medium heat, melt the butter. Add the shallot and garlic and cook, stirring constantly, about 2 minutes. Add the chicken broth and bring to a boil. Add the cream and then whisk in the polenta. Reduce the heat to a simmer, stirring constantly with a wooden spoon. Add the thyme and rosemary and continue to cook, stirring frequently, until the polenta is tender, creamy, and thick, 15 to 20 minutes. Remove from the heat and whisk in 1/4 cup of the cheese. Season to taste with salt and pepper.
Position a rack in the middle of the oven and heat the oven to 400° F. Line a large rimmed baking sheet with parchment paper. Halve the tomatoes lengthwise and hollow them out. Cut a very thin slice off the bottom of each tomato half so the filled tomato can lie flat without rolling. Arrange the tomatoes on the baking sheet and season them lightly with salt. Fill each tomato half with some of the polenta, mounding just slightly (depending on the size of your tomatoes, you may or may not use all the polenta). Sprinkle the remaining 2 Tbs. of cheese and the parsley over each. Bake until the tomatoes are soft, about 20 minutes. Let cool slightly before serving.