Place a large skillet over medium heat. Add 1 tablespoon of butter. Once melted, add orzo and toast, stirring frequently, until lightly golden, about 3 to 4 minutes.
Pour in chicken broth and bring to a boil. Reduce heat to low, cover, and simmer 12 to 15 minutes, or until liquid is mostly absorbed and orzo is tender. Remove from heat and keep covered.
While the orzo cooks, place a second large skillet over medium-high heat. Add the remaining butter. Once melted, add mushrooms in an even layer. Do not stir immediately. Let them cook undisturbed for 2 to 3 minutes to develop browning.
Season the mushrooms with a pinch of the kosher salt. Add the diced onion and stir. Continue cooking until the mushrooms are deeply golden and most of the moisture has evaporated, about 5 to 6 minutes.
Stir in garlic and cook for 30 seconds.
Reduce the heat to medium-low. Add the heavy whipping cream, Parmesan cheese, remaining salt, black pepper, and red pepper flakes (if using). Stir gently and bring to a low simmer (do not boil). Cook 2 to 3 minutes, until slightly thickened.
Stir in lemon juice.
Add the mushroom sauce to the cooked orzo and stir gently until creamy and well combined. Cover loosely to keep warm while preparing the steak.
Wipe out the mushroom skillet so it can be used to cook the steak.
Pat the steak pieces completely dry with paper towels. Season evenly with the salt and pepper.
Heat the vegetable oil in the skillet over medium-high heat until shimmering. Add the steak in a single layer, leaving space between pieces. (Depending on the size of your skillet, you may need to cook in batches.) Do not move the steak for about 2 minutes so a crust forms. Flip and cook another 2 to 3 minutes, or until browned and cooked to your desired doneness.
Reduce the heat to medium-low and add butter. Tilt the pan and spoon the melted butter over the steak for 30 seconds.
Top the creamy mushroom orzo with steak bites and garnish with chopped parsley.
Serve immediately.