Step 1

To prepare BBQ sauce, stir together all ingredients in a small bowl; set aside.

Step 2

To prepare onion rings, divide flour into two equal parts. Mix half of the flour with enough water so that the batter is the consistency of heavy cream. Stir in sea salt. Separate sliced onions rings and toss with remaining sea salt and let stand for 5 minutes. Lightly coat rings with other half of flour.

Step 3

Dip rings into batter and lightly coat with walnuts. Deep fry at 275°F until crispy.

Step 4

To assemble burger, brush bun and burger with olive oil and brown on flat top. Place burger on bun bottom and top with BBQ Sauce, smoked mushrooms, crispy onions and the bun top.