If using freshly shelled peas, cook in a 4- to 5-quart saucepan of boiling water (not salted), uncovered, until tender, 3 to 5 minutes, then drain in a colander.
Briskly simmer cream with 1/4 cup mint in a 1- to 1 1/2-quart heavy saucepan until slightly thickened and reduced to about 1/2 cup, 4 to 5 minutes.
Pour through a fine-mesh sieve into a bowl, pressing on and discarding mint.
Cook celery and onion in butter in 4- to 5-quart heavy saucepan over moderate heat, stirring, until softened, about 4 minutes.
Add cooked fresh or thawed frozen peas and cook, stirring, until heated through, 3 to 5 minutes. Stir in cream, salt, and pepper and briskly simmer, uncovered, stirring occasionally, until peas are well coated, about 1 minute. Stir in remaining 2 tablespoons mint and salt to taste.
