Wash sesame seeds, drain in colander.
Heat a pan and dry roast sesame seeds on medium flame until toasted. When you crush seeds between two fingers, it should easily crush.
Transfer to a plate and cool.
In the same pan dry roast few fenugreek seeds on low flame until seeds change color, just one shade. Let cool.
Grind fenugreek seeds and mustard seeds separately.
Grind sesame seeds to a powder. Grind in 5 second intervals taking care not to over grind, which could result in a sesame paste.
Wash apple and wipe with a kitchen towel.
Cut vertically into quarters and remove the core. Cut each quarter into ¾” pieces. This is about 1 ¾ cup of apple pieces.
Transfer to a wide bowl, add garlic, squeeze lime juice and toss apple pieces. Lime is used to avoid discoloration of apples and also to give some sour taste to the pickle. If apple is very tart, use less lime juice or skip all together. You can always add more lime juice if needed, after tasting the pickle.
In a separate bowl take roasted sesame seeds powder, chili powder, salt, fenugreek powder, mustard powder and mix together to combine. This is a make ahead spice mix.
Add the spice mix to apple pieces and mix well.
In a pan heat oil for tempering.
When oil is hot, add mustard seeds and turn off the heat.
Immediately add fenugreek seeds or fenugreek powder and chili seeds if using.
Seeds will toast in the hot oil.
Let the tempering cool, pour over apple mixture and mix well. The pickle might look dry and oil may not float right away.
Let the pickle settle for 2-3 hours or over night and oil will begin to float on top. If it does not, and if you feel the pickle masala is dry, heat some oil, cool and mix it in the pickle.
Transfer to glass jar and store at room temperature for 3-4 days and then refrigerate.