Combine lemon juice, Dijon mustard, shallot, garlic, 1/2 tsp. salt, and a pinch of pepper in a small bowl. Stir to blend; set aside to let flavors meld. Mix thinly sliced kale and shredded brussels sprouts in a large bowl.
Measure 1/2 cup oil into a cup. Spoon 1 Tbsp. oil from cup into a small skillet; heat oil over medium-high heat. Add almonds to skillet and stir frequently until golden brown in spots, about 2 minutes. Transfer nuts to a paper towel-lined plate. Sprinkle almonds lightly with salt.
Slowly whisk remaining olive oil in cup into lemon-juice mixture. Season dressing to taste with salt and pepper.