Rinse and dry the chilies. (the chilies must be dry, use a paper towel if needed).
Wearing gloves, remove and discard the stems. Then, using a knife, make a slit in the large side of the chili. Set aside for now.
Add the fennel seeds, cumin seeds, mustard seeds, and fenugreek seeds to a small pot or pan over medium heat and dry roast the whole spices for 2 minutes, or until the spices are slightly toasty and fragrant. Grind the spices (you can grind them so that they are coarse, finely ground, or somewhere in-between, it’s fine to leave some larger pieces).
Add mustard oil to a saucepan over medium-high heat and cook for 4-5 minutes, or until you see the oil begin to smoke (you’ll want to turn the stove off as soon as the oil starts to smoke, otherwise your smoke alarm will go off - ask me how I know).
Wait a couple of minutes for the oil to cool down a bit, then add the chilies, freshly ground spices, salt, turmeric to the pot and mix well. Wait 10 minutes, then stir in the lemon juice.
Once the mixture has cooled completely, pour it into a dry glass jar and let it sit for at least 3-4 days before eating. Shake it once a day so the oil coats all the chillis.
Store the achar on the counter for up to a month (shaking it every other day) or in the fridge for up to six months.