Combine first 4 ingredients in a small bowl, stirring well. Spread mustard mixture evenly on 4 toast slices.
Top evenly with bacon, onion, spinach, and remaining bread slices, pressing down slightly.
This sandwich is very easy to make and has a great combo of sweet, savory, salty, and spicy. This relish is also delicious spread on a panini sandwich with grilled, sliced chicken and cheese. —Martine Giddings
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