This hearty dish brings together the best of both potato gratin and seafood casserole. Sustainably sourced smoked Atlantic mackerel, high in heart- and brain-benefitting omega-3s, is paired with a rich creamy sauce, peas, and a mashed potato crust. It’s also a main dish that can be prepared ahead of time and simply reheated in the oven before serving. Swap out the mackerel for fillets of smoked wild salmon, if you prefer.
Pull the pin
If using whole smoked mackerel, be sure to carefully remove any pin bones when breaking apart the flesh. These can present a choking hazard when hidden within the casserole.
This collection was originally published in the October 2025 issue of alive magazine.
Smoked Mackerel Gratin
Ingredients
- 1 1/2 lbs (680 g) russet or Yukon gold potatoes, peeled and cubed
- 6 Tbsp (80 mL) unsalted butter, divided
- 1 3/4 cups (435 mL) whole milk, divided
- 2 tsp (10 mL) Dijon mustard
- 2 Tbsp (30 mL) finely chopped rosemary
- 1/2 tsp (2 mL) salt, divided
- 2 tsp (10 mL) grapeseed or extra-virgin olive oil
- 1 medium yellow onion, chopped
- 8 oz (227 g) cremini mushrooms, sliced
- 1 cup (250 mL) frozen peas
- 4 cups (1 L) spinach
- 2 Tbsp (30 mL) all-purpose flour
- 1 tsp (5 mL) Italian seasoning
- 1 cup (250 mL) ricotta cheese
- 1 lb (450 g) smoked mackerel, flesh broken into 1 inch (2.5 cm) chunks
- Juice of 1/2 lemon
Directions
01
Preheat oven to 375 F (185 C) and place rack in middle of oven. Grease 9 x 13 inch (23 x 33 cm) casserole dish.
02
In large saucepan, place potatoes, cover with water, and simmer until very tender. Alternatively, place potatoes in steamer basket set over 2 inches (5 cm) water and steam until very tender. Drain well and mash with 3 Tbsp (45 mL) butter, 1/4 cup (60 mL) milk, Dijon mustard, rosemary, and 1/4 tsp (1 mL) salt.
03
In skillet, heat oil over medium. Add onion and heat 5 minutes, until softened. Add mushrooms and heat 3 minutes, until softened. Stir in peas and spinach; heat until spinach is wilted.
04
To make sauce, in medium-sized saucepan, heat remaining 3 Tbsp (45 mL) butter over medium. Whisk in flour and cook, whisking constantly, for 2 minutes, until mixture is golden. Whisk in remaining 1 1/2 cups (375 mL) milk and Italian seasoning; bring to a simmer and heat over low, stirring occasionally, for 5 minutes. Whisk in ricotta cheese and remaining 1/4 tsp (1 mL) salt; heat for 2 minutes. If sauce is too thick, whisk in additional milk.
05
In casserole dish, place cooked vegetables and mackerel. Squeeze on lemon juice. Add ricotta sauce to dish. Top with mashed potato and spread out in even layer. Bake for 25 minutes. Switch on oven broiler and heat until potato topping is a couple shades darker and crispy in parts.
Matthew Kadey, MSc, RD, is a dietitian, nutrition writer, and recipe developer. He is also the author of Rocket Fuel (VeloPress, 2016) and Muffin Tin Chef (Ulysses Press, 2012).
