In a small bowl, stir the cornstarch into 1 Tbs. of the chicken broth until smooth. Stir in the remaining 5 Tbs. broth, along with the ponzu and sesame oil.

Season the sliced beef with 1/2 tsp. salt and several grinds of pepper. Heat 2 Tbs. of the oil in a large skillet or stir-fry pan over medium-high heat. When the oil is very hot (it should shimmer), add the beef and stir-fry just until it loses its raw color, 2 to 3 minutes. Transfer the beef to a clean bowl. Return the pan to medium-high heat, add the remaining 1 Tbs. oil, and then add the mushrooms and peas. Stir-fry until the mushrooms are tender and the peas are crisp-tender, 4 to 5 minutes. Add the scallions, ginger, and garlic, and stir-fry for 30 seconds. Return the beef to the pan. Give the broth mixture a quick stir to recombine and then pour it into the pan. Cook, stirring, until the sauce thickens, about 1 minute. Serve immediately.