Heat oil in a saute pan over medium heat. Add onion; cook for 1 minute. Add garlic, curry, ginger, cumin, salt, red pepper flakes, coriander, cayenne pepper, celery seed, tarragon, and cinnamon. Cook until fragrant, 1 to 2 minutes more. Add potatoes and mixed vegetables. Cook, stirring constantly, for 3 minutes. Pour broth into the potato mixture. Reduce heat to medium-low. Cook, stirring occasionally, until potatoes are tender and broth is mostly absorbed, 12 to 15 minutes. Place flour in a medium bowl. Make a well in the center; add salt, oil, and water, in that order. Mix into a stiff dough, roll into a ball, and wrap in plastic wrap. Set aside for 30 minutes. Divide dough into 8 balls. Roll each ball into a 10-inch circle. Cut each circle in half, moisten the edges, and shape each half into a cone around your finger. Fill dough with 1 to 2 tablespoons of the vegetable mixture. Pinch edges shut. Heat oil in a large saucepan or deep-fryer. Deep-fry samosas in batches until lightly browned, 2 to 3 minutes. Serve hot.