In a large pot (I use a dutch-oven), melt the butter with the olive oil over medium heat. Add the garlic and onion and saute until just starting to caramelize, about 4 minutes.
Slice the corn kernels from the cobs and add the kernels to the garlic and onion (don't throw out the cobs!). Cook together until softened, about 4 minutes.
In a saucepan, bring the half and half to a simmer over medium-low heat, add the corn cobs, and simmer until reduced by about half.
Remove half of the corn and onion mixture into a bowl. Reserve for later.
Warm the chicken broth in a separate pot on the stove until just steaming.Stir the rice, thyme sprigs, and 1 tsp. salt into the remaining corn in the pot. Increase the heat to medium-high and toast the rice, stirring, about 4 minutes, until the rice is just barely browning. Pour in the wine. Stir until the wine is absorbed into the rice.
Decrease the heat back to medium and stir in a few ladles of the hot chicken broth. Cook, stirring often, until the broth is absorbed, keeping a gentle simmer going. Continue this process, adding the chicken broth a ladle at a time and stirring until it is absorbed, until all the broth is gone.
Taste a small spoon of rice to check for doneness, it should be soft with just a slight chew to it.
Discard the corn cobs and stir the half and half into the risotto.
When the rice is cooked, stir in the reserved corn and onion, fontina cheese, and freshly cracked black pepper and salt, to taste.
If the risotto is too thick, it can be thinned with a little more broth. If it is too thin, it can be simmered a bit longer to thicken.
Remove the woody thyme sprigs. Serve, swirling 1 tablespoon mascarpone onto each serving and topping with extra thyme, if desired.