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