Bring an 8-qt. pot of lightly salted water (about 1 tsp. salt) to boiling. Add greens, pushing down to submerge. Cook until just wilted, 3 to 5 minutes. Transfer to a large bowl of ice water to stop the cooking, 1 minute. Drain in a colander leaving fairly wet; set aside.
In a large skillet cook bacon over medium heat until crisp. Transfer to a plate lined with paper towels, reserving drippings in skillet. Cool completely and finely chop bacon.
Add shallot to reserved drippings in skillet; cook 3 minutes or until translucent and fragrant. Add garlic; cook 1 minute more. Stir in mustard. Add greens; toss to coat. Add salt, black pepper, and half the bacon; stir to heat through. Squeeze half a lemon over greens; slice remaining half. Top greens with lemon slices and remaining bacon.