Preheat the oven to 425°F.
Add the canola oil to a cast iron pan and heat over medium-high heat.
Once the oil is hot, add the rosemary and garlic and sauté for 1 minute.
Pat the chicken thighs dry with a paper towel and season both sides liberally with salt and pepper.
Add the chicken, skin side down, to the cast iron pan. Top with lemon slices.
Allow the chicken to cook undisturbed for about 10 minutes, until the skin is crispy and is pulling away from the pan. Resist every urge to move the chicken around. The skin gets crispiest if it stays in one spot!
Flip the chicken over (so the skin side is up) and place the whole pan in the oven for 10-15 minutes, until the chicken is finished cooking. The chicken is done when an instant read thermometer shows the internal temperature at 165°F.
When the chicken is finished, remove the cast iron pan from the oven. Be very careful when doing this - the pan gets very hot in the oven and it's quite heavy!
Transfer the chicken to a plate, but leave the rest of the juices and seasonings in the pan.
Pour out the cooking liquid so there is only about 1 tbsp of liquid left in the pan.
Over medium-high heat, deglaze with the white wine. Scrape the bottom of the pan to get the crispy bits off of the bottom.
Once the wine is reduced by half, add the chicken broth. Bring to a simmer, and reduce by half again.
Remove the large pieces of lemon, garlic, and rosemary from the sauce.
Whisk the butter into the sauce until incorporated.
In a separate bowl, mix the cornstarch with 2 tbsp water to create a slurry.
Whisk the slurry into the sauce to thicken it.
Once the sauce is thickened, it is ready to be served.