Season the chicken with the dried thyme, and salt and pepper to taste. Heat a large saute pan over medium heat and add the olive oil. Put the flour in a shallow dish. Working in batches, dredge the chicken in the flour and add to the pan; saute until cooked through, about 3 minutes per side. Transfer to a plate and tent with aluminum foil.
Add the red onion and fresh thyme to the pan and cook over low heat, stirring occasionally, until aromatic, about 5 minutes.
Combine the wine, chicken broth and the juice of 1 lemon in a bowl. Turn the heat under the pan to high and deglaze with the broth mixture, scraping up the pan with a wooden spoon. Cook until the liquid starts to reduce, about 10 minutes. Remove from the heat and whisk in 1 1/2 tablespoons butter. Season with salt and pepper.
Meanwhile, put the spinach in a microwave-safe dish and add 3 tablespoons water; cover loosely with plastic wrap and microwave until hot and wilted, 5 to 6 minutes. Drain and toss with the remaining 1/2 tablespoon butter, the juice of the remaining lemon, and salt and pepper to taste. Arrange on a platter and top with the chicken. Spoon the sauce on top.