• In a fine-mesh strainer, rinse the lentils.

In a small sauce pot, combine the lentils and 4 cups lightly salted water. Bring to a rolling boil over high heat and cook until the lentils are completely tender, 18 to 20 minutes. Drain, transfer to a medium bowl, and let cool.
While the lentils cook, in a small bowl, combine the flaxseed meal and 2½ tablespoons water and stir with a fork; let stand for 10 minutes while you prepare the salad and topping.

• Trim the stem ends from the Brussels sprouts; thinly slice the Brussels sprouts crosswise.
• Cut the apple into quarters and cut away the core; thinly slice the fruit.
• Peel the red onion and cut in half. Thinly slice half for the salad; coarsely chop enough of the other half to measure ⅓ cup for the burgers.
• Cut the avocado in half; remove the pit and scoop out the flesh from half the avocado; save the other avocado half for another use.

In a medium bowl, combine the Brussels sprouts, apple, sliced onion, apple cider vinegar, and 1 teaspoon oil and toss to coat. Season to taste with salt and pepper.
In a small bowl, combine half the avocado and as much sambal oelek as you like. Using a masher or fork, mash the avocado until smooth; season the topping to taste with salt.

• Coarsely chop enough walnuts to measure 2 tablespoons; set aside 1 tablespoon for the salad.

In a medium frying pan over medium-high heat, warm 1 teaspoon oil until hot but not smoking. Add the chopped onion, season with salt and pepper, and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Stir in the carrots and cook until starting to soften, 1 to 2 minutes. Stir in the burger spice blend and as many chile flakes as you like and cook until fragrant, about 1 minute. Remove from the heat.
Using a masher or the bottom of a cup, mash the lentils, leaving some whole. Fold in the onion mixture (wipe out the pan). Stir in the flaxseed mixture and 1 tablespoon chopped walnuts; season with salt and pepper and mix well. When the lentil mixture is cool enough to handle, form into 2 equal balls.

In the same pan used for the onion, warm 1 to 2 teaspoons oil over medium-high heat until hot but not smoking. Add the lentil balls and, using a spatula, gently flatten them into patties. Cook, turning once, until browned and crispy on the outside and hot within, 1 to 2 minutes per side, depending on thickness.
While the burgers cook, prepare the lettuce wraps and finish the salad.

• Trim the root end from the iceberg lettuce and separate the large leaves for the burgers; save the rest for another use.

Add the remaining 1 tablespoon chopped walnuts to the salad and toss to combine.

Transfer the lettuce leaves to individual plates. Top with the burgers and spread with the avocado topping. Serve with the Brussels sprout salad.