1. Bring first 4 ingredients to a boil in a large saucepan. Reduce heat to a simmer, cover, and cook 20 to 30 minutes or until tender, stirring occasionally. Drain and transfer lentils to a bowl. Cool to room temperature, about 20 minutes.
2. Whisk together 2 tablespoons vinegar, lemon juice, 1 teaspoon kosher salt, and pepper in a small bowl. Gradually whisk in oils until blended. Pour over cooled lentils, adjust seasoning, and set aside. Cover with plastic wrap, and chill until needed, up to 1 day in advance.
3. Melt butter in a large sauté pan over low heat. Add onion and remaining 1 teaspoon kosher salt, and cook over low heat, stirring occasionally, 45 to 50 minutes or until soft and caramelized. Transfer to a bowl, and deglaze the pan with remaining 2 tablespoons vinegar. Pour hot vinegar into bowl with onion; add crème de cassis, and stir to incorporate. Let mixture cool 10 minutes, cover with plastic wrap, and chill until needed.
4. Return lentils and caramelized onion to room temperature 1 hour before serving. Arrange crackers on a plate with a bowl of caramelized onion and a bowl of lentils. Let guests spread crackers with 1/4 teaspoon crème fraîche each, and top with a spoonful each of lentils and caramelized onion.