Bring broth to a boil in a medium saucepan over medium-high heat. (If using mustard greens, add to broth.) Meanwhile, pat shrimp dry with paper towels and quarter the lemon. While stirring the broth with a wooden spoon, slowly pour in grits. Reduce heat to medium-low and cook, stirring frequently, until the broth is absorbed and the grits thicken, 5 to 6 minutes. Stir in spinach until wilted, cover and keep warm.

Place bacon in a single layer in a large frying pan over medium heat and cook until browned and crisp, 8 to 10 minutes. Transfer to a paper-towel lined plate. Remove all but 2 Tbsp. bacon fat from pan.

Increase heat to medium-high. In same skillet, add shrimp in an even layer, and sear undisturbed until starting to brown on the bottom, about 2 minutes. Stir in bell pepper, celery, garlic, thyme, smoked paprika, salt, pepper, and cayenne if desired and cook, stirring occasionally, until shrimp are just cooked through and opaque, about 2 minutes more. Remove skillet from heat; add green onions and squeeze the juice from a lemon quarter over shrimp, tossing to combine. Sprinkle with parsley.

To serve, divide grits among 4 shallow serving bowls. Top with shrimp mixture and crumble the bacon slices over the shrimp. Serve with remaining lemon wedges and hot sauce.