How to Make It
In a skillet, toast the fennel seeds and crushed red pepper over moderate heat until fragrant, 1 minute. Transfer to a mortar and let cool completely. With a pestle, grind the spices to a coarse powder; transfer to a bowl.
In the same skillet, cook 3 1/2 tablespoons of the butter over moderate heat until it starts to brown, 2 minutes. Add the 1/4 cup of sage and cook, turning once, until crisp, 2 minutes. With a slotted spoon, transfer the sage to a plate. Pour the browned butter into the bowl with the spices. Repeat with the remaining butter and the 36 sage leaves; reserve the leaves for garnish.
Add the first batch of fried sage leaves to the mortar and crush them with the pestle. Add the crushed sage to the butter along with the oregano, lemon juice and tequila and season with salt. Keep warm.
Light a grill. Line a platter with rock salt. Grill the oysters, flat side up, over high heat until they open, 1 to 2 minutes. Discard the flat top shell and place the oysters on the rock salt, being careful not to spill their liquor. Spoon the warm tequila butter over the oysters, garnish each one with a crisp sage leaf and serve.