In a large bowl, add 1 quart cool water, 1 tablespoon salt, lemon juice, and 1 teaspoon liquid crab boil. Stir to dissolve salt. Add 2 quarts ice and set aside.

Fill a medium saucepan two-thirds full with water, add 1 teaspoon Essence, and bring to a boil. Season shrimp with remaining teaspoon Essence, if desired. Add shrimp to saucepan and return water to a boil. Immediately remove from heat and allow shrimp to sit in hot water until just cooked through, about 1 minute longer. Remove shrimp from hot water and transfer to ice bath until completely cool.

While shrimp are cooling, melt butter in a large skillet over medium-high heat. Add onion, bell peppers, garlic, 1/4 teaspoon salt, and 1/4 teaspoon cayenne and cook, stirring frequently, until vegetables are thoroughly wilted but not browned, about 10 minutes. Transfer vegetable mixture to a large bowl and allow to cool.

Preheat oven to 350 degrees F.

Remove shrimp from ice bath and drain well, then finely chop and transfer to a large mixing bowl. Add cooled vegetable mixture, cream cheese, mayonnaise, pepper Jack cheese, green onions, thyme, black pepper, 1/4 teaspoon salt, and 1/4 teaspoon cayenne and stir to combine. Taste and adjust seasoning if necessary. Transfer mixture to two oval gratin dishes, each about 7 by 10 1/2 inches, using the back of a spoon to evenly spread mixture. Sprinkle cracker crumbs in an even layer over top of dip. Bake, uncovered, until dip is heated through and golden brown on top, 25 to 30 minutes. Remove from oven and allow to cool for at least 15 minutes before serving. Serve warm, with French bread croustades for dipping.