Add the onion, red bell pepper, and garlic to a food processor. Process until finely chopped.
Add drained tuna, egg, almond flour (1/2 cup), salt, and pepper. Process until well combined.
Heat olive oil (2 tablespoons) in a cast iron (or other heavy) skillet over medium-high heat.
Form patties with approximately 1/4 cup of the tuna mixture and fry until golden brown on both sides (about 2-3 minutes on each side), flipping carefully (as they will have a tendency to fall apart when they are warm).
Place on plate to cool for at least 5 minutes before serving.
Meanwhile, to make the aioli, add all ingredients to food processor and process until well combined.
Serve tuna croquettes with aioli.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.