Peel and finely dice the shallots. Place the frozen peas in a heatproof bowl, cover with boiling water, then drain.
In a pan over a moderate heat, add the butter and cook the shallots for 3-4 minutes to soften.
Add the peas and the crème fraîche and season well with salt and plenty of freshly ground black pepper. Heat through for 1-2 minutes. Roughly blend with a stick blender or, alternatively, roughly mash the peas with the back of a fork. Serve warm.