Heat oil in the pressure cooker over medium-high heat and stir in beef cubes. Cook until the beef is browned on all sides; remove beef and set it aside. Drain and discard excess grease.
Lower the heat to medium and add onion; cook and stir until onion has softened and turned translucent, about 5 minutes. Stir in garlic and mushrooms and continue to cook about 5 minutes more, until garlic is fragrant but not brown.
Pour in beef stock, beef stew meat, and any accumulated drippings; season with salt and pepper. Return to high heat, seal the lid, and bring to full pressure.
Reduce the heat to low, maintaining full pressure, and cook for 20 minutes.
While the stew is cooking, bring a large pot of lightly salted water to a boil. Stir in noodles and cook until tender, stirring often, about 8 minutes. Drain and set aside.
Remove the pressure cooker from the heat and allow the pressure to drop naturally. Dissolve cornstarch into 1/2 cup of cold water. When the pressure has dropped, remove the lid and whisk in the cornstarch mixture. Bring the beef and mushrooms back to a boil and cook, stirring constantly, for 2 minutes or until the sauce has thickened. Pour the beef over the noodles and serve.
