Add the olive oil to a large pot. Dice the onion and saute the onion in the olive oil over medium-low heat. While the onion is cooking, thinly slice the mushrooms. Add them to the pot and continue to saute. Mince the garlic, add it to the pot, and continue to saute until the mushrooms and onions have released all of their water and become limp. There should be no water pooled on the bottom of the pot.
Add the butter to the pot and allow it to melt. When it is completely melted, add the flour. Stir the flour and butter together with the onions, mushrooms, and garlic. The mixture will form a thick paste. Continue to stir and cook this mixture for 3-4 minutes, or until it begins to coat the bottom of the pot and turn slightly golden brown.
Warm the milk slightly in the microwave (or in a separate pot if needed). Slowly whisk the milk into the pot with the flour mixture, making sure to dissolve all of the flour stuck to the bottom of the pot. Turn the heat up to medium-high and allow the sauce to come up to a simmer, at which point it will begin to thicken. Make sure to whisk the mixture occasionally as it heats. Season the sauce with salt, pepper, and thyme. Once the sauce is thick enough to coat a spoon, turn the heat off and set it aside.
Bring a separate large pot of water to a boil over high heat. Add the pasta and continue to let it boil for 7-10 minutes, or until the pasta is tender. Drain the pasta, then return it to the pot with the heat turned off.
Begin to preheat the oven to 350 degrees. Thaw the spinach and squeeze as much moisture out of it as possible. Roughly chop the olives. Add the spinach, olives, drained tuna, sour cream, and a 1/2 cup of grated parmesan to the pot with the drained pasta. Pour the prepared mushroom sauce over top and stir everything together until evenly combined.
Lightly coat an 8×8 inch casserole dish with non-stick spray. Pour the pasta mixture into the dish. If desired, sprinkle the remaining 1/4 cup of parmesan cheese over top. Bake the casserole in the fully preheated oven for 20 minutes, or until it is hot, bubbly, and the parmesan begins to turn golden brown. Divide into 9 portions and then serve.