In a medium, heat-safe bowl, mix together flax seeds through boiling water. Let soak 15 minutes. Cover a 12x16-inch baking sheet (or larger) with parchment paper. Pour mixture in the middle, then evenly spread to edges with a spoon or spatula. Sprinkle with flaked or coarse sea salt.
Heat oven to 350 F. Place on middle rack for 40 to 50 minutes — cracker edges will curl slightly with browned edges and will be stiff when parchment is lifted off baking sheet. Let cool for a minute or two, then carefully remove parchment and break cracker into pieces. (If some pieces bend, continue baking those pieces in 5-minute increments until stiff.) Store in an airtight container at room temperature up to 2 weeks.
Per serving: 37 calories, 3 grams fat (0 grams saturated fat), 2 grams total carbohydrates, 0 milligrams cholesterol, 115 milligrams sodium, 1.5 grams fiber, 1.5 grams protein.
