Add bajra flour, ajwain, black pepper, cumin if using, red chili powder, turmeric, sesame seeds, and salt to a bowl. Mix until the spices look evenly distributed.
Add the oil or melted ghee. Rub into the flour with your fingertips until the mix looks like damp sand and holds a clump when squeezed.
Add warm water 1 tablespoon at a time. Mix and press until you get a semi-stiff dough that just comes together. It should not feel soft like roti dough. Bajra absorbs water as it rests, so stop while it still feels firm.
Knead gently 1 to 2 minutes to smooth the surface. Cover and rest 20 to 30 minutes. Preheat the oven to 350°F (180°C) and line a baking sheet with parchment.
Roll, cut, and prick
Place the dough between two sheets of parchment. Roll to about 1/4 inch thick (a little under 1/2 cm). If the edges crack, press them back together and keep rolling. Dust with a pinch of bajra flour only if the dough sticks.
Cut into thin rectangles about 2 inches long (or small squares). Gather scraps once, press together, and roll again.
Prick each mathri several times with a fork on both sides. Arrange on the baking sheet with a little space between pieces.
Bake until crisp
Bake 8 to 10 minutes, flip each mathri, then bake 8 to 12 minutes more until the edges are golden and the centers feel firm. Total time is usually 16 to 22 minutes depending on thickness.
Cool on the sheet 2 to 3 minutes, then move to a wire rack and cool completely before storing in an airtight jar.
Notes
Semi-stiff dough wins: too much water gives soft centers. Add water slowly and let the rest finish hydrating the flour.Roll thin and even to about 1/4 inch; prick both sides so they dry through.Cool fully before jarring so warm mathri do not steam and soften.Vegan: use oil instead of ghee; keep the same fat amount.Jowar swap: use jowar (sorghum) flour 1:1; flavor is milder; watch color rather than the clock.Re-crisp softened mathri at 300°F (150°C) for 5 to 8 minutes, then cool before sealing.Store in an airtight jar at room temperature up to 1 to 2 weeks once fully cool and crisp.