Bring a large pot of water to a rolling boil. Add the spinach then push the leaves down to submerge them in the water. Blanch until the stems are no longer rigid (about 40 seconds) and then drain the spinach and dump it into a bowl of ice and cold water to stop the cooking immediately (this is called shocking the veggie). Once cooled, squeeze the excess water by wrapping the spinach in a towel and squeezing it. Set this aside.
Dump the water out of the stockpot you used for the spinach, then refill it with 6 quarts of water and 3 tbsp of salt. Bring to boil on medium-high heat. Start the sauce while waiting. Once water is boiling, add fettuccine and cook according to package directions.
Melt the butter in a saucepot on medium-high heat. Once hot, but not burnt, add the onions and saute for 5 minutes, or until translucent and soft. Add the garlic and saute for 2 minutes more.
Lower the heat to medium and add the flour. Combine by whisking so the flour coats the onions and garlic and creates a roux. It will look clumpy and that's ok! Let it cook for 2-3 minutes until there is a golden-brown color to the roux.
Add the heavy cream, and whisk to break up the roux. Keep whisking until the roux is broken up. This should take you about 2 minutes. Allow it to thicken, 5 minutes, whisking often. Once thickened, add the chicken broth and 3/4 cups of milk. Whisk often and simmer for 10 minutes. Start the chicken while watching the sauce. Once it starts to thicken up, add the rest of the milk, whisk to combine, and simmer for 10 minutes more. It should be creamy enough to coat a spoon now, while not being overly thick. (*1)
For the chicken: Season the chicken breasts with salt and pepper to your liking. Heat a grill pan (*2) on medium-high heat and spray with cooking spray. Cook the chicken in the grill pan for 7 minutes on one side. You want to make sure there are grill marks before flipping. Flip, then grill for 5 minutes more, or until the internal temperature is 165F.
Continue the sauce: Add the spinach, artichokes, parmesan cheese, garlic powder, salt, and pepper and stir to combine.
Serve immediately.