Preheat oven to 375°. Combine flour, flaxseeds, garlic salt, and baking powder in a large bowl. Using your fingers, work butter into flour mixture until mixture resembles cornmeal. Gradually add milk, stirring with a wooden spoon until a crumbly dough forms (dough will be on the dry side but moist enough to hold together). Shape dough into a ball, wrap in plastic wrap, and transfer to the refrigerator to let chill for 10–15 minutes.
Divide dough in half. Wrap 1 dough half in plastic wrap to keep it from drying out. Roll other piece of dough out on a lightly floured surface to a 1⁄8"-thick rectangle, about 10" × 12". Trim off ragged edges. Brush dough with half the oil and sprinkle with half the salt. Cut dough into 15 rectangles about 1 1⁄2" × 5". Using a thin metal spatula, transfer dough rectangles to a parchment paper–lined baking sheet about 1⁄2" apart and set aside. Repeat process with remaining dough, oil, and salt, making 30 crackers in all.
Bake crackers until golden brown, about 10 minutes, rotating pans halfway through baking time. Transfer crackers to a wire rack to let cool completely.