Heat olive oil in a 5-quart pot over medium-high heat. Add beef in a single layer; cook, turning once, until just browned on both sides, 5 to 7 minutes. Transfer to a plate using tongs; cover with aluminum foil and keep warm.
Combine butter with the drippings in the pot and melt over medium heat. Add mushrooms and onion; cook and stir until mushrooms brown and onion softens, 7 to 9 minutes. Stir in water and bring to a simmer. Add 1 tablespoon plus 1 teaspoon soup base; mix well.
Place sour cream in a bowl. Ladle in 1/2 cup of the simmering mushroom-soup base mixture. Beat with a whisk to combine. Set aside until the final step.
Stir cream of mushroom soup and egg noodles into the pot and mix well. Bring to a boil and reduce heat. Simmer, uncovered, stirring occasionally, until noodles are tender yet firm to the bite and liquid is mostly reduced to sauce, 10 to 15 minutes.
Stir cooked beef and any accumulated liquid into the sauce. Return to a simmer. Remove from heat; stir in sour cream mixture. Add peas. Let sit until heated through, 5 to 7 minutes. Top with parsley.