Preheat the oven to 375°F.
Heat the olive oil in a large skillet over medium heat until shimmering. Add the onion and sauté with a healthy pinch of salt until just translucent, about 2-3 minutes. Stir in the garlic and sauté until fragrant, about 2 minutes more. Add the mushrooms and a little more salt and sauté until the mushrooms are tender, about 5-6 minutes. Toss in the artichoke hearts and frozen peas and sauté until the peas are thawed and warmed through, about 3 minutes. Take the vegetables off the heat and set aside.
While you're sautéing the vegetables bring a large pot of water to a boil. Salt the water and cook the pasta shells until just al dente, about 2-3 minutes shy of the advised cook time on the package. Drain the shells and rinse them under cold water. Separate the shells and set aside.
Melt the butter in a large saucepan over medium heat until frothy. Whisk in the flour to form a roux. While whisking constantly, pour in the wine and the milk in roughly 1/4 cup increments. Wait until each addition is fully integrated before adding more liquid. Wait for bubbles to break the surface, then stir in the cheese and Dijon mustard. Stir until the cheese is completely melted. Remove the sauce from the heat and stir in the yogurt, fresh tarragon and lemon juice. Taste and season with salt accordingly.
Pour half of the sauce in a large mixing bowl. Stir the sautéed veggies, the tuna and the spinach into the sauce and set aside. Take the other half of the sauce and spoon a little into the bottom of a greased 9x7 inch casserole dish. Using the back of the spoon, smooth the sauce to form a thin even layer along the base of the dish. Take a cooked pasta shell and spoon some of the tuna mixture into the shell until it's filled to the top. Place the stuffed shell on top of the layer of sauce in the casserole dish. Repeat until all the shells are filled.
Pour the remaining plain sauce over top of the shells and sprinkle with mozzarella cheese and crushed saltines. Cover the shells with foil and bake for 30 minutes. Remove the foil and turn the oven to broil. Broil the shells for 5 minutes or until the cheese is browned and bubbling. Remove the casserole from the oven and let stand for 10 minutes before serving.