Cook the potatoes in a large saucepan of salted, boiling water until tender; usually about 15 minutes. Drain and leave to cool to one side.
Blanch the grean beans in a large saucepan of salted water for 2 minutes, then remove and blanche the mangetout in the same water for 1 minute. Refresh both kinds of beans in iced water.
Parboil the onions in a saucepan of salted, boiling water for 3-4 minutes until just tender, then remove and refresh in iced water. Parboil the carrots in the same saucepan of salted, boiling water for 3-4 minutes until just tender, then refresh in iced water.
Bring the stock to a simmer over a moderate heat then whisk in the crème fraîche. Season to taste and keep over a low heat.
Preheat the grill to hot.
Brush the scallops with the olive oil and season. Grill in batches; cook for 2 minutes on one side until golden and slightly crisp on top, then flip and cook the other side for 1 minute.
Meanwhile, whisk the butter into the sauce until melted. Arrange a mixture of the vegetables in serving bowls, then pour the sauce on top. Sit the grilled scallops on top and serve immediately.