Start a large pot of generously salted water to a boil. Add the pasta and boil until al dente (check a few minutes before the package instructions indicate). When cooked, drain the pasta. If it finishes before the sauce, toss with a drizzle of olive oil to prevent sticking.
Meanwhile, heat a large saute pan or saucepan over medium low heat. Melt the butter, then add the garlic and thyme and cook for 30 seconds to 1 minute until the garlic is lightly browned and fragrant. Add the milk, heavy cream and Parmesan cheese and cook for 2 to 3 minutes until the cheese has melted and it thickens slightly. Add the sun dried tomatoes, spinach, and salt and cook 2 to 3 minutes more, until the spinach is wilted.
In a small bowl, mix the flour with 2 tablespoons water. Pour it into the pan with the sauce, then stir until it thickens. Cook 2 minutes more.
Add the drained pasta and stir to combine. Stir in a few grinds of fresh ground black pepper if desired. Serve immediately. Leftovers last refrigerated for up to 3 days.