Meanwhile, preheat oven to 350°. Spread almonds on a sheet pan and toast in oven about 9 minutes, turning once, until fragrant and golden. Set aside to cool.
Place garlic and 2 cups cold water in a large saucepan. Bring to a boil over high heat; strain, discarding water. Repeat. Return garlic to pan; add potatoes, 1 tsp. salt, and enough cold water to cover. Bring to a simmer over medium-high heat and cook about 12 minutes, or until potatoes can be easily pierced with a knife. Drain off cooking liquid, reserving 1/2 cup. Return potato mixture to saucepan; add yogurt and 1 tsp. salt. Using a potato masher, mash until creamy, adding 1/4 cup olive oil and reserved cooking water by tablespoonfuls. Whisk until fluffy and smooth (take care not to overwork). Adjust seasoning to taste.
Preheat grill to high heat. Grill fish about 1 minute per side. Lower heat to medium and grill 3 to 5 minutes per side, or until fish flakes easily with a fork. Alternatively, preheat broiler; position top rack 5 inches from heat. Place fish on foil-lined pan; cook 7 to 10 minutes.
Meanwhile, in a food processor, grind almonds to a coarse meal. Add orange zest, 2 Tbsp. olive oil, and 1/4 tsp. salt; pulse to combine. If necessary, add a bit more olive oil until a smooth, nearly pesto-like texture is achieved.
To serve, spoon mashed potatoes onto the centers of 4 dinner plates. Top with grilled fish and a drizzle of almond pesto.