Pat diced chicken breast cubes completely dry with paper towels. In a medium bowl, toss chicken with Italian seasoning, paprika, onion powder, 3/4 tsp salt, and black pepper until evenly coated.
Heat 1 tbsp olive oil and 1 tbsp butter in a 12-inch cast iron skillet over medium-high heat until shimmering and butter foams.
Add chicken cubes in a single uncrowded layer (sear in two batches if necessary). Sear undisturbed for 3 to 4 minutes until golden-brown crust forms, then flip and cook 2 minutes more until just cooked through (165 F). Transfer chicken to a warm plate.
In the same hot skillet, add remaining 1/2 tbsp olive oil. Arrange zucchini slices in a single layer and sear undisturbed for 2 to 3 minutes until lightly charred and caramelized. Flip and cook 1 minute more until crisp-tender. Season with remaining 1/4 tsp salt and transfer to the plate with the chicken.
Reduce heat to medium-low. Add the remaining 2 tbsp butter and minced garlic to the skillet. Sautu00e9 for 30 to 45 seconds until fragrant and golden (watch closely so garlic does not scorch).
Pour in chicken broth and fresh lemon juice, using a wooden spatula to scrape up all flavorful caramelized browned bits (fond) from the bottom of the pan. Simmer for 1 minute to reduce slightly into a glossy sauce.
Return the seared chicken bites and caramelized zucchini back into the skillet. Toss thoroughly for 1 minute to coat every piece in the velvety garlic herb lemon butter.
Remove from heat. Sprinkle with fresh chopped parsley and freshly grated lemon zest. Serve immediately with warm crusty bread, fluffy basmati rice, or buttery pasta.