In a skillet over medium to low, add vegetable oil and let it heat up.
Add the piece of beef with shallow cuts and fry this meat until both sides are seared.
Remove the meat from the skillet and cut it into cubes.
In a large pan over medium to low heat, add the unsalted butter and stir until fully melted.
Add all the beef cubes and fry them until golden brown.
Remove all the meat from the heat and set aside.
In the same pan, add the onion cut into slices, the red bell pepper cut into smaller pieces and sauté until the onion wilts.
Then, add the meat again.
Season with the garlic powder, the salt, a pinch of black pepper, the sweet paprika and mix to combine.
Add the soy sauce, the mustard, the water and mix everything together once more.
Cover the pan and let everything cook for another 15 minutes.
After this time has passed, dissolve the cornstarch in the water.
Add this mixture to the pan and mix everything for another 3 minutes.
Turn off the heat, add parsley and green onion to taste and mix once more to combine.
Serve.