Mix the Cajun spice blend in a small bowl and set aside.
Pat the shrimp dry with paper towels, then toss them in half of the spice mix until evenly coated. Let sit for 10 minutes to absorb the flavors.
Heat your skillet over medium-high heat until it shimmers, then add the oil, swirling to coat the surface evenly.
Once the oil is hot and just begins to smoke, carefully add the shrimp in a single layer. Sear for 2-3 minutes until edges turn pink and slightly charred.
Flip the shrimp using tongs or a spatula, cooking for another 2 minutes until fully opaque and cooked through, with a nice sear on the edges.
Add the minced garlic to the pan and cook for 30 seconds until fragrant, making sure it doesn't burn.
Squeeze the fresh lemon juice over the shrimp and stir gently to coat everything evenly. The citrus brightens the smoky spices and adds a lively zing.
If using, add the butter and swirl it into the pan for a glossy, rich finish.
Remove the skillet from heat and toss the shrimp in the remaining spice mix if desired for extra flavor. Let rest for 2 minutes to allow juices to settle.
Transfer the Cajun shrimp to a serving platter, garnish with sliced green onions, and serve immediately with lemon wedges and your favorite sides.