Getting Ready: Center a rack in the oven and preheat the oven to 325 degrees F. Butter a 10 ½-inch fluted tart pan with a removable bottom and put it on a baking sheet lined with parchment or a silicone mat.
Toss blueberries into a small pan of boiling water and steep for a minute, then drain and pat dry.
Whisk the polenta, flour, baking powder, and salt together.
Working with a stand mixer fitted with the whisk attachment or with a hand mixer in a large bowl, beat the ricotta and water together on low speed until very smooth. With the mixer at medium speed, add the sugar, honey, and lemon zest and beat until light. Beat in the melted butter, then add the eggs one at a time, beating until the mixture is smooth. Reduce the mixer speed to low and add the dry ingredients, mixing only until they are fully incorporated. You’ll have a sleek, smooth, pourable batter.
Pour about one third of the batter into the pan and scatter over the blueberries. Pour in the rest of the batter, smooth the top with a rubber spatula, if necessary, and dot the batter evenly with the chilled bits of butter.
Bake for 35 to 40 minutes, or until a thin knife inserted into the center of the cake comes out clean. The cake should be honey brown and pulling away just a little from the sides of the pan, and the butter will have left light-colored circles in the top. Transfer the cake to a rack and remove the sides of the pan after about 5 minutes. Cool to warm, or cool completely.