Combine butter and onion in 1 -quart glass casserole.
Microwave on high for 3 minutes or until onion is partly cooked.
Add mushrooms, frozen peas, salt, pepper and allspice.
Recover, and continue cooking on high for 4 minutes.
Stir and continue cooking on high for 3 to 4 minutes or until peas are tender-crisp.
Let stand, covered, 3 minutes before serving