To make the vinaigrette, in a large serving bowl, whisk together olive oil, lemon juice, garlic, Dijon mustard, thyme, and salt and pepper to taste (I like 1/4 teaspoon salt and 1/8 teaspoon pepper). Set aside.
To cook the farro, in a medium saucepan, heat olive oil over medium-high heat until shimmering. Add farro and cook, stirring until fragrant, about 2-3 minutes.
Add water and salt to taste (I like 1 teaspoon) and bring to boil. Reduce heat to low, cover, and simmer until farro is tender, about 15-18 minutes.
Drain through a fine-mesh sieve, then spread farro out in a single layer on a rimmed baking sheet to cool to room temperature.
To the serving bowl with the vinaigrette, add cooled farro, peas, arugula, dill, and salt and pepper to taste (I like 1/2 teaspoon salt and 1/4 teaspoon pepper). Toss to combine and sprinkle with feta.
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Notes
Farro: Look for semi-pearled or pearled farro (not whole farro). These types have some or most of the grain removed and the quickest cooking time.
Yield: This recipe makes about 10 cups of salad, enough for 10 (1-cup) servings (or 5 larger, 2-cup servings if serving for lunch).
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Make ahead:Cook and chill the farro and prep the dressing. Store the ingredients in the refrigerator separately until ready to serve. Then toss everything together when you need it.