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1
Melt 1 tablespoon butter in a 5-quart pot over medium heat, then cook bacon and onion, stirring occasionally, until most of bacon fat is rendered, about 5 minutes. Add sauerkraut and wine and simmer briskly, uncovered, until most of wine has evaporated, about 3 minutes.
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2
Stir in stock and heavy cream, then simmer gently, uncovered, stirring occasionally, 20 minutes.
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3
Remove from heat and stir in mustard, sour cream, and remaining 2 tablespoon butter. Puree soup in batches in a blender until smooth and creamy (use caution when blending hot liquids), then return to pot. Season with salt and pepper. Keep warm.
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4
Mix flour, spices, and kosher salt in a bowl. Pat scallops dry, then generously coat one side of each scallop evenly with spice mixture.
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5
Heat oil in a 12-inch heavy nonstick skillet over medium-low heat until it shimmers. Cook scallops, plain side down first, turning once, until just cooked through, 5 to 6 minutes total. Reserve spiced cooking oil.
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6
Divide soup among 6 bowls. Place 2 scallops in each bowl and top with arugula. Drizzle soup with reserved oil.
