Pour the buckwheat into a pan and sprinkle in a little salt. Cover with 150ml boiling water and pop on a lid. Return to the boil, then reduce the heat to a simmer and cook for 8 mins.
Trim any woody ends away from the sweet baby broccoli. Thickly slice the mushrooms. Peel the garlic, and slice as thinly as possible. Trim the roots and top 3 cm off the spring onions. Cut roughly into 2-3cm pieces.
Warm a large pan to a medium-high heat. Add the butter and allow to melt, then slide in the broccoli, mushroom slices, chopped spring onions and garlic. Sprinkle in a little salt and pepper. Fry for 4-5 mins, stirring occasionally, till tender and golden.
Drain the buckwheat if necessary, then tip into the pan with the vegetables. Squeeze in the lemon juice. Toss together and warm through for 1 min.
Scatter the salad into a salad bowl and toss through the rocket leaves. Divide onto plates and serve.