In a saucepan, combine the farro and 2cups (16fl oz/500ml) water. Bring to a boil over high heat, then reduce the heat to low and simmer, covered, until the grains are plump and tender to the bite, about 30 minutes. Remove from the heat, uncover, and let cool slightly. (The farro can be prepared up to 1 day in advance and stored in the refrigerator.)
Make the vinaigrette
Meanwhile, in a large bowl, whisk together the lemon juice, ¼teaspoon salt, and a pinch of pepper. Gradually whisk in the oil until well blended.
Assemble the salad
Add the radishes, cucumber, mint, and farro to the vinaigrette and toss to coat evenly. Gently toss in the spinach and cheese and serve.