Place the lentils in a medium pot of water. Bring to a boil. Reduce the heat and simmer, uncovered, stirring occasionally, for 17 to 20 minutes, or until tender but not mushy. Drain.
Bring a large pot of salted water to a boil and set a bowl of ice water nearby. Drop the snap peas into the boiling water and blanch for 1 to 2 minutes, or until tender but still bright green. Use a slotted spoon to scoop the snap peas out of the boiling water and into the ice water. Chill for 1 minute, then drain and transfer to a kitchen towel to dry. Chop into 1-inch pieces and set aside.
In a large bowl, whisk together the shallot, lemon juice, olive oil, salt and several grinds of pepper. Add the lentils and toss to coat. Transfer to a serving platter or meal prep containers and top with the snap peas, cucumbers and radish slices.
When ready to serve, drizzle with the green tahini and garnish with mint leaves.