Rinse lentils well and remove any shriveled lentils or debris. Place lentils in a small pot, and cover with two cups of water. Bring to a low boil, reduce heat to a simmer, and cook uncovered for 25-30 minutes. Start tasting lentils at 20 minutes and every few minutes after to stop cooking when they are done.
When lentils are done, stir a teaspoon of salt into the pot. Drain, and rinse with cold water.
While lentils are cooking, chop your cucumber and parsley. In a medium-sized bowl, whisk together 1/2 teaspoon salt, olive oil, lemon juice, and sriracha. Gently stir in cucumber, parsley, and drained lentils until coated. Store in refrigerator until ready to serve.
Before serving, drain off excess liquid, and then stir in feta cheese if desired.