Preheat oven to 400º. In a medium baking dish, combine tomatoes, shallots, garlic, and all but about 2 tsp. oil; season with salt, black pepper, and red pepper flakes (if using), then toss to combine.
Place two-thirds of feta in center of tomato mixture. Drizzle with a bit of oil that collected in baking dish. Bake until tomatoes are bursting and feta is golden on top, 40 to 45 minutes.
Meanwhile, on a small baking sheet or baking dish, toss bread, ¼ tsp. salt, and reserved 2 tsp. oil; set aside.
Carefully transfer tomato mixture, along with broth, to a blender. Blend until smooth, then transfer to a medium saucepan. If using an immersion blender, carefully transfer tomato mixture to a medium saucepan and blend until smooth, then stir in broth.
Bring to a simmer over medium-low heat and cook, stirring occasionally, until reduced, about 10 minutes.
Meanwhile, bake reserved bread, tossing halfway through, until golden brown and crispy, 10 to 15 minutes.
Ladle soup into bowls. Top with croutons, dill, and remaining feta.