1 pound filet mignon, cut into 1-inch pieces

1½ teaspoons kosher salt, divided

¼ cup soy sauce

¼ cup oyster sauce

¼ cup granulated sugar

½ teaspoon freshly ground black pepper

Juice of ½ lime

1 tablespoon grapeseed oil

2 tablespoons unsalted butter

¼ medium red onion, thinly sliced

1 bunch scallions, sliced into 1½-inch pieces

2 cups baby greens (such as spinach, arugula or watercress)