Rinse the rice under cold water and drain. Bring 7 cups of water to a boil in a 4-quart pot over high heat. Add 3/4 tsp. salt. Add the rice, reduce the heat to a simmer, and cook uncovered, stirring occasionally and adding more boiling water as necessary to keep the rice covered, until tender, about 20 to 25 minutes. Drain and rinse the rice with cold water to stop the cooking.
Transfer the rice to a foil-lined rimmed baking sheet, drizzle with 1 Tbs. of the olive oil, and toss lightly to coat. Spread the rice on the baking sheet and cool completely at room temperature or in the refrigerator.
Put the vinegar in a small bowl and gradually whisk in the remaining 1/2 cup of oil. Taste and season with salt, pepper, and additional vinegar or oil as needed.
Put the cooked and cooled rice in a large serving bowl and toss to break up any clumps. Add the apples, grapes, celery, lentils, Cheddar, almonds, parsley, and 1/2 cup vinaigrette and toss. Taste and season as needed with more vinaigrette, salt, and pepper, and serve.
The salad can be refrigerated for up to 1 day. If making ahead, let sit at room temperature so it’s not refrigerator-cold and season with more vinaigrette, salt, and pepper before serving.