Add the oats, quinoa, and millet to a large, fine-mesh strainer and rinse for about a minute under running water; set aside.
Heat 1 tablespoon oil in a 3-quart saucepan over medium heat.
Add the rinsed grains to the oil and cook for 2-3 minutes or until they begin to smell toasted, then stir in the water and salt.
Bring to a boil, then cover, turn down the heat, and simmer for 20 minutes.
Turn off the heat and let sit for 5 minutes, then remove the lid and fluff with a fork.
Spread the hot grains onto a large, rimmed baking sheet and let cool for at least half an hour, then transfer to a large bowl.
In a separate medium bowl, whisk together the remaining 1/2 tablespoon oil, orange juice, honey, yogurt, cinnamon, and vanilla.
Pour the yogurt mixture into the grains and stir until well coated. Season with additional salt to taste.
Cover and refrigerate overnight.
Top with peaches, blueberries, and sliced almonds before serving; garnish with an additional drizzle of honey if desired.