Melt butter and olive oil in a wide, shallow saucepan over medium heat. Add onion and saute, stirring occasionally, until it begins to soften. Add pancetta and continue sautéing, until the onion is soft but not browned and the pancetta is just starting to caramelize. Add garlic and saute for 2 minutes more Stir in rice and stir to coat with fat from the pan.
Add one ladle of chicken stock at a time, stirring continuously between each addition, until most of the liquid has been absorbed. About halfway through, add wine along with one of the additions of broth. It will take about 35 minutes total to cook the rice al dente. It will be thick and creamy when it’s done cooking, and the rice will have a chew, without an unappealing “raw” center.
Remove risotto from heat. Add Parmesan, basil and thyme. Season with pepper as well as salt, if needed, and stir to combine. Cover and let cool thoroughly in the refrigerator.
Heat oil in a large Dutch oven over medium-high heat. Place the breadcrumbs on a plate, season with salt and pepper and mix to combine. Stir beaten egg into reserved, cooled risotto. Using generously heaped tablespoons, form rice into individual patties, inserting a nugget of mozzarella in the center. Take care to completely enclose the cheese so it doesn’t ooze out. Coat with breadcrumbs, shaking off any excess prior to frying.
Working in batches, fry arancini until golden flipping halfway through, 3 to 4 minutes per side. Drain on paper towels. Serve warm with marinara sauce.