Bring a large pot of water to a boil, salt it, add the pasta and cook until al dente. Drain.
While the pasta is working, in a medium saucepan, melt the butter over medium heat. Whisk in the flour and stir for a minute, then whisk in the milk and cook, stirring frequently, until the sauce begins to thicken, about 5 minutes. Season to taste with salt, pepper and nutmeg. Keep warm.
In a large, deep skillet, heat the EVOO, 2 turns of the pan, over medium-high heat. Add the artichokes, celery, shallots and garlic, and salt and pepper to taste; cook until tender, 6 to 7 minutes. Add the tuna, breaking it up with a fork. Stir in the peas to warm through. Add the pasta and sauce and toss to combine. Top with the parsley, thyme and lemon peel.