Pat chicken breasts dry and season both sides with about 1/2 tsp of the sea salt, 1/4 tsp of the black pepper, and the garlic powder.
Heat a large non-reactive skillet over medium heat and add 1 Tbsp butter and 1 Tbsp olive oil until melted and shimmering.
Add the chicken to the hot skillet and sauté 3–5 minutes per side, or until the internal temperature reaches 165°F; remove to a cutting board and let rest, then slice if desired.
In the same skillet over medium heat add the remaining 1 Tbsp olive oil and the sliced mushrooms; sauté 5–7 minutes until lightly golden and excess moisture has evaporated.
Add the chopped sun-dried tomatoes, chopped green onion, minced garlic, the remaining 1/2 tsp salt, and remaining 1/4 tsp pepper; sauté about 1 minute until the garlic is fragrant.
Stir in the heavy cream and sprinkle the Parmesan over the top; bring to a light boil while stirring, then reduce heat and simmer about 2 minutes until the sauce is slightly thickened.
Add the spinach and stir just until wilted, then return the sliced (or whole) chicken to the skillet and spoon the warm sauce over it; remove from heat.
Taste and adjust seasoning with more salt and pepper if needed, then serve.
