Pour peas in a saucepan and cover with water. Bring water to a boil, reduce heat to medium-low, and simmer until peas are hot and tender, 5 to 7 minutes; drain.
Cook and stir bacon in a large skillet over medium-high heat until the fat renders and the bacon is crisp, 7 to 10 minutes. Stir onion into the bacon dripping; cook and stir with the bacon until tender, 5 to 7 minutes.
Pour cream over the bacon mixture; cook and stir until the cream thickens, about 5 minutes. Season thickened cream with monosodium glutamate, salt, and pepper.
Pour sherry into the thickened cream.
Add drained peas and mushrooms to the skillet; gently stir to coat.