1. Preheat the oven to 425 degrees.

To Make the Lentils:

1. Combine the lentils and water in a saucepan over medium-high heat. Bring to a boil and then reduce the heat until the liquid is gently bubbling.
2. Cook lentils until they are barely tender, about 20 to 25 minutes. 
3. When ready to use, drain the lentils, spread them out on a clean dish towel and pat them dry.
4. On a baking sheet, mix the cooked lentils with another 1/4 teaspoon of the salt, 1 teaspoon of the oil and all the lemon zest.
5. Transfer the sheet to the oven (you can cook at the same time as the radish mixture), and cook both until the lentils are crisped and starting to blister in places, and 

To Make the Radishes:

1. Cut the radishes and potatoes into halves (or quarters, if larger) to get them roughly the same size.
2. Toss them on a large, rimmed baking sheet with 1 tablespoon of the oil, half the lemon juice, the honey and 1/4 teaspoon of the salt.
3. Roast for 15 minutes until the radishes and potatoes are soft and starting to brown at the edges, about 15 minutes. Shake the pan once or twice during cooking, as needed.

To Make the Buttermilk Drizzle:

1. While the lentils and the radishes are cooking, whisk together the buttermilk, the remaining lemon juice and the remaining two teaspoons of oil in a liquid measuring cup. Add the remaining 1/4 teaspoon of the salt and the pepper. Taste, and add more salt as needed.

To Assemble:

1. Toss the lentil, potato and radish mixture with spinach (or arugula) leaves on a large platter.
2. Drizzle the buttermilk dressing over the top, and serve warm or room temperature.