Line a 9 x 13 inch (23 x 33 cm) pan with plastic wrap. Set aside.
Heat vegetable stock in a medium saucepan over medium high heat until it’s steaming.
Melt butter in a large saucepan over medium heat. Add arborio rice to the saucepan and toast the rice until it’s translucent, about 4 - 6 minutes. You don’t want the rice to take on any color, so turn down the heat if you see browning.
Add white wine and cook until the rice has absorbed all the wine. Add in tomato paste and stir until combined with the rice. Ladle ¾ cup (175 ml) hot stock into the large saucepan, just until the rice is covered. Once the rice has absorbed about half of the stock, add in another ½ cup (120 ml) ladle of hot stock. Continue this process until the rice is tender, but still has a bite to it. From the time you add in the first ladle of hot stock, it should take about 16 minutes to fully cook the rice. The risotto will be slightly soupy once fully cooked.
Remove the risotto from heat. Season with parmesan cheese, salt, and pepper.
Transfer rice to the plastic wrap lined pan, spread into an even layer, cover with additional plastic wrap, and refrigerate until the rice is firm, about 3 hours.
Add flour to a shallow bowl. In a separate bowl, whisk together eggs and water. Add panko into another shallow bowl.
Cut cooled risotto into 12 even sized pieces. Wrap the risotto around a piece of mozzarella cheese to form a small log. Coat the risotto in flour, then dip into the egg, followed by the panko crumbs. Repeat until all supplì are coated.
Heat canola oil to 330°F (165°C) in a heavy bottom saucepan.
Fry in small batches, about 5 - 7 minutes, flipping halfway through the cooking process, until deep golden brown. Be sure to maintain a consistent oil temperature throughout the cooking process by checking with an instant read thermometer.
Using a slotted spoon, transfer to a baking sheet lined with paper towels. Optionally, sprinkle with additional kosher salt. Allow the supplì to rest for 5 minutes so the cheese can fully melt inside. Serve immediately.