Preheat oven at 180 C.
In a large pan, bring 2 litres water to a boil with 1 tsp salt.
In a mixer / food processor pulse the peanuts till coarsely crumbed.
Meanwhile, scrub the sweet potatoes well with a scrubber and lots of water to remove all the soil. Do not peel. Finely slice them, and add them to the boiling water. Add the peas too.
Check in 4 minutes, if they are partially cooked. If yes, drain the water in a colander and keep aside the veggies.
In a saucepan, heat butter. Add flour and make a 'roux' until all the flour is absorbed in the butter and turns lightly aromatic. Add the milk with constant whisking. If a few lumps remain, it's okay with this dish. Add the salt, dried rosemary, pepper and nutmeg.
Lightly grease a mid-sized baking dish. Place the partly cooked sweet potato slices and peas, gently flattening them down. Sprinkle some pepper over the top.
Add the sauce, lightly smoothening it out with a spatula.
Top with peanut crumbs.
Place in the middle of the oven for 30 minutes or so.
Serve warm with a salad or a soup.