Tip the rice into a small pan and pour in 150ml boiling water. Add a pinch of salt, cover and bring to the boil. Turn the heat right down and very gently simmer for 25 mins till the rice has absorbed the water and is tender. Take the pan off the heat and leave the rice in the pan, lid on, for 2-3 mins to steam it and finish cooking.
While the rice cooks, halve the avocado and scoop out the stone. Peel away the skin and chop the flesh into small chunks. Slide the avocado into a bowl. Peel the short cucumber (the peel can be bitter) and cut the cucumber into chunks to match the avocado. Peel and finely chop the red onion. Add the cucumber and onion to the avocado.
Finely chop the coriander, leaves and stalks. Peel and finely chop the garlic clove. Slide them into the bowl. Grate in the lime zest and squeeze in the juice. Pour in 1 tsp olive oil and add a little salt and pepper. Stir well till combined and set aside.
Preheat your grill to high. Trim the ends off the flat beans and slice into bite-sized pieces. Halve the cherry tomatoes. Scatter the tomatoes and beans onto a baking tray. Drizzle with 1 tsp oil and sprinkle over a little salt and pepper and a pinch or two of cayenne pepper (cayenne pepper is hot, so add as much or little as you prefer). Toss to mix. Slide the veg under the grill for 5 mins.
Use a spatula to turn the veg over. Sprinkle a little salt and pepper over the plaice fillets and lay them on top of the veg. Slide the fish and veg back under the grill for a further 5 mins, till the fish is cooked and the veg are tender and slightly charred.
Divide the rice between a couple of plates and top with the plaice and grilled veg. Spoon over the avocado salsa and serve.