Heat 2 tbsp olive oil in a large skillet over medium-high heat. Cook chicken for 8-10 minutes until golden and no longer pink. Remove and set aside.
Add the remaining 1 tbsp olive oil, onion, garlic, thyme, salt, and pepper to the skillet. Cook for 1-2 minutes until onions soften. Stir in Dijon mustard and cook for 30-60 seconds.
Add the chicken broth, heavy cream, and orzo. Stir and bring to a boil, then reduce heat to medium-low. Simmer for 10-15 minutes, stirring often, until orzo is al dente and sauce thickens.
Stir in lemon juice, lemon zest, spinach, and Parmesan. Cook for 1-2 minutes until spinach wilts.
Return the chicken to the skillet and mix well. Remove from heat. The sauce will thicken as it cools.
Enjoy this creamy, zesty, one-pot meal!