Add the flour, milk, eggs, & dried thyme to a high-speed blender, seasoning with 1 teaspoon kosher salt & ground black pepper as desired. Blend on high speed, stopping to scrape down the sides as needed, until the mixture forms a smooth & frothy batter. Set aside to rest for 10-15 minutes while the oven & skillet preheat.
Once oven & skillet are preheated, carefully remove the hot skillet from the oven. Place the melted butter in the skillet, swirling to coat the bottom in the pan. Pour prepared Dutch baby batter into the skillet. Return the skillet to the oven. Bake 15-20 minutes, until the Dutch baby is puffed, the edges are set, & the top is as golden brown as you like. Remove from the oven for serving.
Melt 1 Tbsp of the butter in a medium skillet over medium-high heat. Add the sliced asparagus, seasoning with ½ S&P as desired. Cook, stirring occasionally, until the asparagus is vibrantly green & tender, 3-4 minutes. Remove from the heat & squeeze ½ of the lemon juice over top. Meanwhile, melt the remaining 2 tablespoons butter & whisk with the juice of the remaining ½ lemon. Set aside.
Spoon the cooked asparagus into the center of the savory Dutch baby, then top with the cooked asparagus, thinly sliced smoked salmon, & crumbled cheese. Finish with a drizzle of the prepared lemon butter sauce & a sprinkling of finely chopped fresh herbs.