Preheat the oven to 425 degrees .
In a saucepan, heat the stock over medium; cover and keep warm.
Arrange the shrimp on a baking sheet. Toss with 2 tbsp. olive oil and the crushed garlic; season with salt and pepper. Arrange the lemon, cut sides down, on the baking sheet.
Invert a small bowl in a large bowl. Place one corncob upright on top of the small bowl. Cut the kernels off the cob (they'll fall into the big bowl). Using the back of your knife, scrape to get the milky juice from the cob. Repeat with the other ear of corn.
Heat a round-bottom pan or large skillet over medium-high. Add the remaining 2 tbsp. oil, two turns of the pan; add the onion and chopped garlic and cook, stirring often, until softened, about 3 minutes. Add the rice, thyme and bay leaf; stir 1 minute. Add the wine; stir until evaporated, 1 to 2 minutes. Season the rice. Add the stock a few ladles at a time, stirring with each addition. (From the time you begin adding the stock, the risotto will take exactly 18 minutes.) When the risotto is a little more than halfway done, put the shrimp in the oven; roast until opaque in the center, 7 to 8 minutes. When 5 to 6 minutes remain in the cooking time for the risotto, stir in the corn and its juices. When the risotto is done, remove the bay leaf, stir in the cheese and butter; season.
Douse the shrimp with the roasted lemon. Top the risotto with the shrimp and herbs.