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1
Taste the raw mango before cutting, because flesh that is only mildly sour will produce a flat, lifeless pickle that no amount of spice can rescue — you want an intensity that makes you wince. Cut the mango into large pieces, retaining the stone (gutthi, గుట్టి) in each piece where possible, as the stone carries its own sharp savour into the oil. Pat the pieces completely dry, since a single drop of water in the jar can turn the pickle rancid within days.
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2
Dry-grind the mustard seeds freshly to a coarse powder, so the volatile oils are released at the moment of grinding and carry their full sharp, nose-clearing heat into the pickle rather than having already dissipated as they do in pre-ground commercial powder. The freshly ground mustard should hit the back of the throat immediately — if it does not, your pickle will be mild and short-lived. Grind in short pulses to keep the texture coarse, not fine.
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3
In a large, scrupulously dry bowl, combine the freshly ground mustard powder, Guntur red chilli powder, turmeric, and salt. Add the mango pieces and toss thoroughly so every cut surface and the area around each stone is completely coated in the spice and salt mixture — this is where the dish lives or dies, because uneven coating leaves unseasoned flesh that neither preserves nor flavours correctly, and that cannot be corrected once the jar is sealed. Every surface must be densely, uniformly red before you proceed.
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4
Pour the sesame oil over the coated mango and mix to distribute, then pack tightly into a clean, dry jar, pressing down so the oil and released mango juices begin to rise over the pieces and form a seal that keeps air away from the fruit. Seal and leave at room temperature; the pickle is ready to eat after 3 days, when the salt has drawn sufficient moisture from the mango to form a brine and the cut surfaces have lost their raw, harsh edge. The flavour deepens and rounds further over several weeks.