1/2 pound cucumbers, preferably a small English cucumber, peeled
Kosher salt
1 tablespoon sesame seeds
1/2 teaspoon cumin seeds
2 tablespoons plain (full- or reduced-fat) yogurt or water (see headnote)
1 1/2teaspoons mustard oil
1/8 teaspoon fenugreek seeds
1/8 teaspoon nigella seeds
1 green cayenne chile, slit lengthwise and seeded
1/4 teaspoon cayenne
1/8 teaspoon turmeric
1 tablespoon fresh lemon juice
2 to 3 tablespoons minced coriander leaves
Cut the cucumber lengthwise in quarters. Slice off the seeds (unless the cucumbers are very small and tender) and discard, then cut into 1 1/2 -inch lengths. Cut lengthwise in half again if the pieces are fat. Place in a colander, sprinkle on about 2 tablespoons kosher salt, and set over a bowl or in the sink to drain for 15 minutes.
Meanwhile, in a heavy skillet, dry roast the sesame seeds until golden. Transfer to a plate and set aside. Dry roast the cumin seeds until touched with color and aromatic, then transfer to a spice/coffee grinder, add the sesame seeds, and grind to a powder. Place in a small bowl and stir in the yogurt or water to make a paste. Set aside.
Rinse the cucumbers thoroughly with cold water. Squeeze them gently to squeeze out excess water and place them in a bowl. Add the spice paste and rub the cucumbers all over to coat them. Set aside.
Heat the oil in a small skillet over medium heat. Add the fenugreek, nigella, and chile and cook for about a minute, stirring occasionally, until the spices are aromatic. Add the cayenne and turmeric, stir, and pour the flavored oil over the cucumbers; toss gently. Add the lemon juice and toss, then set aside for 10 to 20 minutes to allow the flavors to blend.
Just before serving, add the coriander leaves and 1/2 teaspoon salt, or more to taste, and toss gently to mix.