Wash and dry all produce. Peel potatoes, then dice into 1/2-inch cubes. Place in a medium pot with enough salted water to cover by 1 inch. Bring to a boil and cook until easily pierced by a knife, 10-15 minutes. Drain.

Trim, then finely chop scallions. Mince or grate garlic. Halve lemon; cut one half into wedges. Pat cod dry with a paper towel, then cut into 1/2-inch cubes. In a small bowl, combine mayonnaise, a pinch of garlic, and a squeeze of lemon. Season with salt, pepper, and more lemon and garlic (to taste, but save a little garlic for the potatoes). Set aside.

Melt 1 TBSP butter in pot used for potatoes over medium-low heat. Add scallions and remaining garlic. Cook, tossing, until scallions are softened and garlic is fragrant, 1-2 minutes. Return potatoes to pot and mash with a potato masher or fork until smooth. Season with salt and pepper. Remove from heat.

Add cod to mashed potatoes in pot along with 2 tsp Cajun spice (we sent more). Stir to combine. Add panko to a shallow dish. Season with remaining Cajun spice, salt, and pepper. Using wet hands, shape cod mixture into 4 equallysized cakes. Press cakes into panko, turning to coat all over. TIP: Planning ahead? Let the cakes chill in the fridge until firm, 15-30 minutes.

Heat 1 TBSP oil in a large pan over medium-high heat (use a nonstick pan if you have one). Add cod cakes and cook until browned and crisped, 5-7 minutes per side, flipping them over carefully. TIP: Add another drizzle of oil if the pan seems dry.

Toss arugula, a drizzle of oil, and a squeeze of lemon juice in a medium bowl. Season with salt and pepper. Divide cod cakes between plates, then dollop with aioli. Serve with salad on the side and lemon wedges for squeezing over.