Combine everything but the chicken to make a marinade.
Add chicken halves and marinate at least two hours, turning once to make sure marinade gets on both sides.
Grill over hot coals or under broiler.

INSTANT POT UPDATE
I tried this with my Instant Pot with great results using chicken breasts.
I had 3 BIG chicken breasts which totaled about 2.8 lbs.
Once the chicken marinated, I dumped everything into the instant pot and set it for 12 minutes on pressure cook.
When it was done I only let it self vent for about a minute before manually venting it.
Meanwhile, I had started my grill and let it heat up.
I moved the breasts right from the pot to the grill, which was not easy as they were almost fall-apart tender.
Got them to a nice finish on the grill.
While it was grilling, I scooped out as much of the lemons and onions as I could get, leaving just the liquid in the pot.
I turned the pot to sauté and added a roux of butter and flour to thicken the sauce a bit.
Once the chicken was ready and the sauce had thickened, I strained the sauce onto the chicken by ladling it through a fine mess strainer.
It was awesome.

 

I decided to sous vide it for 3 1/2 hours at 145°F (65°C), then grilled it to get that char flavor.

If you want to make a sauce like in the Instant Pot version, pour everything out of the sous vide bag into a sauce pan and follow the same instructions as for the Instant Pot version.