To prepare the white bean dip, preheat your oven to 400F. Drizzle the olive oil over the garlic head. Wrap the garlic in aluminum foil and place on a baking sheet. Roast the garlic for 35-40 minutes, or until its all soft and golden. Place the white beans, lemon juice, tahini, salt, thyme, and pepper to taste in the bowl of a food processor fitted with the S blade. Squeeze the roasted garlic out of the garlic head, into the bean mixture (all of it!). Pulse the ingredients a few times to combine. With the motor of the processor running, drizzle in the water. Continue processing until the white bean spread is very smooth. Check the seasoning and adjust seasonings to taste.
Lower the oven to 375F and lightly oil or spray a 9" springform pan (alternately, you can use a 9" tart shell). Bring the broth or water for the polenta to boil in a medium pot. When the liquid boils, pour in the polenta or cornmeal in a thin stream, whisking constantly. Reduce the heat to a simmer. Continue stirring the mixture frequently, until it starts to thicken and bubble (about 15-20 minutes, but it could be more or less). It'll spatter angrily, so watch your hands! When the mixture is very thick and pulling away from the edges of the pot, stir in the salt, nutritional yeast, and pepper. Pour the polenta into the pan or tart shell and allow it to rest for 15 minutes. Transfer it to the oven and bake for 20 minutes, or until it's golden brown on top and pulling away from the baking dish. Allow it to cool for 10-15 minutes.
While you wait for the polenta cooking liquid to boil, half the cherry tomatoes and toss them in the olive oil. Transfer them to a parchment or foil-lined baking sheet. Nestle the garlic cloves between the tomatoes. Sprinkle the tomatoes with a dash of salt and pepper, as well as the rosemary. You can transfer the tomatoes to the 375F oven at the same time as the polenta crust. Bake them for 30 minutes, or until they're caramelized and crisping at the edges.
When all of the components are ready, spread the polenta crust with about 1 heaping cup of the white bean spread (an inverted spatula makes this easy work). Arrange the tomatoes on top, transferring over some of the crispy rosemary leaves. Cut the tart into wedges, and serve. Tart leftovers will keep for up to three days in an airtight container in the fridge. Any leftover white bean spread will also keep for 3-4 days, and can be used as an alternative to hummus.