Bring lentils and 4 cups of water to a boil.
Reduce to a simmer and cook for 30 minutes, until lentils are just tender.
Drain immediately and run with cold water.
Using a coffee/spice grinder, grind mustard seed, black pepper and cayenne pepper to a fine powder.
Transfer the spice powder to a bowl and add boiling water. Stir to combine.
Add lemon juice and slowly drizzle in olive oil, while whisking constantly.
In a large bowl, combine lentils, bell pepper, cilantro and salt.
Add dressing and stir to combine.
Serve chilled or at room temperature.