Heat olive oil in a large skillet over medium-high heat for one minute. Trim chicken, then season one side of the chicken with half of the sea salt, pepper, and rosemary.
Add chicken thighs to the pan seasoned side down, and season the second side while they're in the pan.
Cook chicken for about six minutes per side, or until browned and cooked through.
While the chicken is cooking, chop your veggies.
Once the chicken is cooked through, remove it to a separate plate. Add the onion to the same skillet, reduce heat to medium, and saute for a minute, scraping up the browned bits from the bottom of the pan.
Add mushrooms and continue to saute for about seven minutes, stirring occasionally, or until mushrooms are browned and softened.
Stir in the chopped spinach, garlic, sea salt, and crushed red pepper, and continue to saute over medium heat until spinach is wilted, stirring it into the mushrooms as it cooks down.
Remove the spinach-mushroom mixture from the pan into a separate bowl, and add the chicken back in.
Reduce heat to medium-low. Top each piece of chicken with Havarti (cutting the slices to fit) and cook for just a couple of minutes until the cheese is just melted and chicken is warmed through.
Serve the cheesy chicken thighs topped with the mushroom-spinach mixture.