Heat a deep-sided frypan over a medium-high heat. Add olive oil, onion and garlic and cook until the onion begins to soften and the garlic becomes aromatic. Pour in the chicken stock and bring to the boil, then as soon as it is bubbling away, add the peas.
While the peas are cooking, heat a second pan over a medium heat and season the trout fillets lightly with salt and pepper. Cook the fillets skin-side down until the skin is brown and crispy. Turn and cook for only a few seconds on the other side.
At the same time, simmer the peas until the stock is reduced and they are just cooked. Toss in the knob of butter and swirl the pan until the butter has emulsified with the chicken stock to make a creamy sauce. Add the chopped mint to the pan, give it one more swirl and then taste-test the seasoning, finishing with a big squeeze of lemon juice.
Transfer the trout fillets to serving plates and top with the buttery, braised peas and toasted almonds.