Bring a stainless steel pan to high heat with some oil and add in the chicken.
Toss around for a minute or two until just cooked through, the remove from the pan. (it will finish cooking later)
Add in the onion and sauté for one minute.
Once the pan starts to form brown bits on the bottom, add in the butter, soy sauces, and Mirin.
Add in the zucchini, then after a minute add in broccoli and cook the veggies until the broccoli is couple minutes away from being done.
At this point the veggies and chicken are all a couple minutes away from being cooked through. There should be a good amount of liquid in the pan from the veggies.
Remove the veggies, leaving the liquid still in the pan.
Reduce the liquid down until it forms a thicker soy glaze, then add back the veggies and chicken.
Add in the lemon juice, season with kosher salt, and toss round for a couple minutes until the veggies and chicken are cooked through and coated in the glaze.
Whisk Yum Yum Sauce ingredients together and serve with chicken and veggies.