In a large shallow bowl, whisk together the ¾ cup granulated sugar and 2 teaspoons of cinnamon to make the cinnamon-sugar. Set aside.
In a deep bowl, beat together the eggs, milk, ¼ teaspoon cinnamon, and 1 tablespoon of sugar until well combined.
Slice the two ends off the loaf of bread. Then slice the loaf into four equal pieces. In the top of each slice, use a knife to make a slit about 3” long across the top of the piece and deep enough to reach the bottom crust of the bread without slicing through the bottom crust.
Peel each banana and slice it in half lengthwise and then across to form 4 pieces from each banana.
Stuff two of the banana slices vertically into each of the four prepared bread slices.
Fill a heavy stockpot or Dutch oven with at least 4 inches of oil. Using a candy thermometer to continually monitor the temperature, heat the oil to 375°F.
While the oil is heating, submerge one of the prepared bread slices into the egg/milk mixture. Soak for 1-2 minutes, then turn and soak it another 1-2 minutes on the other side. The thickness of the slice and structure of the sourdough should hold up to soaking for quite some time.
When the oil is hot, use a pair of tongs to lift the bread from the batter, allowing the excess batter to drip off, and then carefully transfer the bread into the hot oil.
Fry for 1-2 minutes on each side, flipping the slice when it is golden brown. When the tonga toast is golden brown all over, lift it from the hot oil and blot it on paper towels to remove oil drips.
While the toast is still warm, set it in the bowl of cinnamon sugar and toss to coat it.
Continue battering, frying, and coating with cinnamon sugar for all four pieces of Tonga toast.
Serve warm with maple syrup and/or strawberry compote or jam.
Notes
As with all deep frying, a Dutch oven works well to keep the oil at as steady of a temperature as possible. You do need at least 4” of oil here because the slices of bread are so thick. The toast will float to the top of the oil, so it is important that at least half of the slice stays submerged in the oil while cooking (and then it can be flipped to cook the other half).It worked well to be soaking the next piece of stuffed bread in the egg batter while the previous one cooks. Soaking each piece for about 5 minutes seemed to allow the batter to permeate through most of the slice, making a rich, delicious piece of french toast. You can use other types of french bread, but sourdough really stands up to the battering and the frying, while holding the bananas securely in the middle.