Wash and dry chillies for a day, under the sun if possible(there should not be any trace of moisture). Also clean and dry a large glass bottle.
Powder the mustard seeds in a spice blender. Combine mustard seed powder and turmeric with salt and keep aside.
Slit green chillies and stuff with the spice paste. Place one by one into the glass jar.
Keep in the sun for a day.
Next day, heat mustard oil to smoking point, let cool till warm and pour on top of the green chillies.
Keep the bottle in the sun for 2-3 days, your pickle is ready.