You can use ketchup in stir fries the same way you’d use hoisin sauce or oyster sauce. Breaking it down, ketchup is tomato, vinegar, salt and sugar. I put it in after I’ve gotten the wok hei I want off the rice/veg/meat/egg, immediately after getting the food and fond off the sides of the wok with a lil shaoxing wine. Add in your sauces, toss to mix, sprinkle with scallion/chive/garlic scape greens, sesame seeds, serve.