In a large skillet, saute mushrooms in butter. Set aside.
Stir the condensed cream of mushroom soup and milk into the skillet until it is blended; at a medium heat, bring mixture to a slow bubble, turn off heat, stir in the instant rice and cover. Let sit for at least 5 minutes.
While the soup mixture is warming, thaw the peas in the microwave at 30 second intervals. Do not overheat the peas.
When the rice is tender, stir mushrooms and peas into the rice.