Peel the potatoes and cut into 3cm chunks. Quarter the carrot lengthwise and cut into 3cm chunks. Slice the garlic.
Step 2 — Cook vegetables
Put the potato, carrot and garlic in a medium saucepan of cold salted water and bring to the boil. Reduce the heat to medium and cook for 10-12 mins until tender. Drain, then return to the pan and cover to keep warm.
Step 3 — Finish prep
Meanwhile, strip the thyme leaves, discarding the tough stems. Finely grate the zest of half the lemon*, then juice the half. Bring a small saucepan of water to the boil for the peas.
Step 4 — Cook fish
Pat the fish dry with paper towel, rub with 2 tsp olive oil and season with salt and pepper. Heat a small frypan over medium-high heat until hot. Cook the fish, skin-side down, covered, for 3 mins. Turn and cook, uncovered, for 4 mins or until cooked through (cooking times may vary depending on thickness of fillets). Remove from the pan and reserve the pan.
Step 5 — Cook peas and make mash
While the fish is cooking, add the peas to the pan of boiling water, return to the boil and cook for 1 min. Drain. Mash the potato, carrot and garlic until smooth. Add 10g butter and 2 tbs milk, season with salt and pepper and stir until combined. Cover to keep warm.
Step 6 — Get ready to serve
Melt 20g butter in the reserved pan over medium-low heat. Cook the thyme and lemon zest, stirring, for 1 min or until fragrant. Remove from the heat. Stir in 1 tsp lemon juice* and 2 tsp extra virgin olive oil and season with salt and pepper. Divide the mash, fish and peas among plates. Spoon over the thyme-lemon mixture to serve.