Place a large, heavy skillet over medium-high heat and add the first olive oil and butter. When the butter has melted and begins to sizzle, add the potatoes and brown slowly and patiently for 15-20 minutes, turning them as necessary until they have evenly browned on all sides.
Meanwhile finely chop or puree the garlic. Whisk the lemon, second olive oil, garlic and oregano into a small bowl and whisk until emulsified. Put browned potatoes into a salad bowl and toss with garlic dressing, salt and pepper and spinach.
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