Nasi Hayashi yang Mudah

Bahan-bahan (4 porsi)

• 1½ onions ((12 oz, 340 g))
• 4 cremini mushrooms ((4 oz, 113 g))
• 10 oz thinly sliced beef (such as ribeye)
• 1 Tbsp unsalted butter
• Diamond Crystal kosher salt
• freshly ground black pepper
• 1¾ cups water
• ½ box Hayashi Rice Sauce (Roux) ((for 4 servings, I use half a 5.6 oz, 160 g package))
• 4 servings cooked Japanese short-grain rice