How to Make It
Place oats in a food processor. Pulse until ground, about 10 seconds. Transfer to a bowl. Add egg, tuna, mustard, 2 teaspoons parsley, lemon rind, salt, pepper, and garlic powder. Mix to combine.
Fill a 1/3-cup dry measuring cup with tuna mixture. Invert onto work surface; gently pat into a 3/4-inch-thick patty. Repeat with remaining tuna mixture.
Heat oil in a large skillet over medium. Add tuna cakes to pan; cook 3 to 4 minutes on each side, until bottoms are golden.
Arrange arugula on a plate. Toss with 1 tablespoon lemon juice. Place tuna cakes over arugula. Add hummus and remaining 1 tablespoon lemon juice. Garnish with chopped fresh parsley, if desired.