Preheat the oven to 350°F. Spray a deep dish 9 inch tart pan or pie pan with cooking spray. Fit the crust into the pan. Set aside.
Slice the tomatoes ¼ inch thick and place side by side in a single layer on doubled paper towels.
Sprinkle lightly with coarse sea salt and gently press another paper towel on top to cover them. Leave the tomatoes to release some of their juices while you prepare the rest of the ingredients.
Melt 3 Tbsp of butter. Toss with the cracker crumbs and shredded Parmesan cheese.
Cook the green onions in the remaining 1 Tbsp of butter until they begin to soften. Mix together the mayonnaise and basil pesto.
To assemble: Spread half of the cracker and Parmesan mixture onto the bottom of the crust and sprinkle one third of the Fontina cheese on top.
Arrange the first layer of the tomatoes over the cheese and spread one half of the basil pesto mayonnaise evenly over the tomatoes.
Sprinkle the next one third of cheese and the cooked green onion over the tomato slices.
Repeat, Parmesan cracker crumbs, tomato slices, basil mayonnaise and end with fontina cheese on top. Season the top with black pepper.
Bake for 38-40 minutes.
Let the tart rest on the counter for 10 minutes before cutting.
Serve warm or at room temperature.