Butterfly your chicken breasts and lay them flat. Sprinkle the salt, pepper, onion powder, paprika, and lemon pepper over the chicken.
Heat the olive oil in the base of a stew pot on medium-high heat. Place in the chicken, seasoned side down, and cook on both sides for about 4 minutes each. Use a lid on the pot to help the steam cook the chicken.
Transfer the chicken to a cutting board to cool, leaving all excess juices in the pot. Turn the heat down to very low and melt in the butter. Once the butter is melted, stir in the pesto sauce and garlic. Stir all your spinach into the pot, getting all the leaves coated in the sauce. Cook on a low heat, continually stirring, until the spinach is almost completely wilted.
Temporarily turn off the heat. Stir in the chicken broth and heavy cream. Then, use an immersion blender to blend the soup until just small pieces of spinach remain, as pictured.
Return the soup to a high heat, place a lid on the pot, and bring to a boil. While the soup comes to a boil, finely chop or shred the chicken into small, bite-sized pieces.
Once the pot is at a boil, stir the chicken back into the stew. Reduce the stew to a simmer and let the pot simmer for about 10 minutes – long enough to let the flavors marry. Serve hot!