Preheat your oven to 400°F and line a baking sheet with parchment paper.
Peel and cube the sweet potato, then steam or boil until fork-tender, about 10-12 minutes.
Finely mince the raw cod or pulse briefly in a food processor until it reaches a ground consistency.
In a mixing bowl, combine the minced fish, beaten egg, salt, pepper, and a pinch of lemon zest, then form into two even patties.
Toss the asparagus with half the olive oil and roast in the oven for 10-12 minutes until tender.
Heat the remaining olive oil in a non-stick skillet over medium-high heat and sear the fish cakes for 3-4 minutes per side until golden brown and cooked through.
Drain the sweet potatoes and mash them until smooth, seasoning with a pinch of salt.
Plate the fish cakes alongside the sweet potato mash and roasted asparagus, finishing the dish with the fresh lemon juice.