Heat the oven to 400°F. Line a baking sheet with parchment paper and set aside.
Warm olive oil in a medium saucepan over medium-high heat. Add onion and sauté until soft and translucent. Stir in the rice and toast, 2 minutes. Add chicken stock 1 cup at a time, ensuring the stock is mostly absorbed before adding more. Add thyme, parmesan, and butter and stir until creamy.
Spread the risotto on a non-lined baking sheet and cool to room temperature.
Arrange three small bowls in a row: one with eggs, one with flour, and one with breadcrumbs and panko.
Scoop enough risotto to coat each provolone cube, covering the cheese completely. Roll into balls, then dredge in flour, followed by the egg wash and the breadcrumb mixture. Place each ball on the parchment-lined sheet and chill in the refrigerator for 30 minutes.
Bake arancini in the oven for 20 to 25 minutes, flipping once during cooking, until golden brown and crispy