Fill a medium-large pot with the oil (a Dutch oven works great!). The oil should rise at least 2 ½ inches high within the pot and easily accommodate three biscuits at a time. Heat the oil to 375°F (yes, a thermometer is a must!).
Meanwhile, line a cooling rack with paper towels. Toss the cinnamon and sugar together in a bowl and set aside.
Once the oil reaches the correct temperature, use metal tongs or a slotted spoon to carefully place two the three biscuits into the pot. Fry the biscuits for 2-3 minutes on each side. Once done, carefully remove the donuts with a slotted spoon (allowing excess oil to drip back into the pot) and let drain on the paper towels.
Be sure to check and adjust the temperature of the oil as you go. It should always be between 360 to 375°F.
After the donuts have cooled and drained slightly, toss them in the cinnamon-sugar mixture. Set aside and continue with the remaining biscuits.
Once all of the donuts have been made, it is time to fill them. Use a chopstick or wooden dowel to slightly hollow out the center of each donut. Do not poke the chopstick out the other side.
Fill a piping bag fitted with a bismarck tip or round piping tip with the cooled strawberry-rhubarb filling. Place the tip in the end of each donut and gently fill the centers. Do not over-fill.
Strawberry-Rhubarb Filling
Place the rhubarb and strawberries in a saucepan over medium heat. Add the sugar and cook until the fruit begins to break down and juices begin to simmer, about 5 to 10 minutes.
Meanwhile, dissolve the cornstarch in the lemon juice. Stir the lemon juice mixture into the rhubarb and cook for a few minutes. Remove the mixture from the stove and blend with an immersion blender until smooth. Cover with plastic wrap and chill in the refrigerator until thickened.