Shell the peas and blanch them in boiling water for 2 minutes. Strain, and reserve their liquor. Melt the butter slowly in a pan, then put in the chopped onion. Simmer for 5 minutes, then put in the peas, salt, mint leaves, and a very little of the pea liquor.
Boil the spaghettini rapidly in plenty of salted water for 4 or 5 minutes, strain and put into deep soup plates. Spoon the peas into the centre of the pasta on each plate, with some of the sauce, and crown each centre with a curl of butter. No parmesan with this.