Mix cracker flour and salt in a bowl. Cut in cold butter until pea-size pieces remain, stir in 2 tablespoons ice water and add reserved water only if needed to make the dough hold together. Flatten into a disk, wrap and refrigerate for 20 minutes.
Melt soup butter in a 6-quart pot and cook diced onion for 5 minutes. Add potatoes, broth, thyme, soup salt and pepper and simmer for 15 minutes until potatoes are very tender.
Meanwhile, heat the oven to 400°F. Roll the chilled dough between parchment sheets to 1/8 inch thick, cut into 12 small triangles and place on a lined baking sheet. Bake for 15 to 18 minutes until golden and crisp.
Blend the potato soup base smooth with an immersion blender, keeping the blade submerged. Add carrots and pearl onions and simmer gently for 10 to 12 minutes until tender, stirring often along the bottom.
Stir in peas, roast turkey and milk and heat gently for 5 to 8 minutes until the turkey reaches 165°F. Keep the soup below a hard boil.
Divide soup among 6 bowls and serve 2 pastry crackers with each, placing them against the bowl rim or alongside just before eating.