POSITION oven racks in top and bottom thirds of oven. Preheat to 325F. Take out 2 large baking sheets.
TOAST sunflower seeds, flax seeds, sesame seeds and poppy seeds in a large frying pan over medium-high, stirring often, until sesame seeds are light golden, 7 to 8 min. Cool slightly in a large bowl.
STIR in flour, oats, onion, garlic and salt to toasted seeds. Gradually pour in water and olive oil while stirring. Place half of dough mixture between 2 large pieces of parchment. Roll out thinly to 12×10-in. Remove top parchment and lift dough on parchment onto a baking sheet. Repeat with remaining dough. Bake in top and bottom thirds of oven, rotating and switching sheets halfway through, until browned and crisp, 50 to 55 min.