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.
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.
Melt butter in a large saut 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 crme de cassis, and stir to incorporate.
Let mixture cool 10 minutes, cover with plastic wrap, and chill until needed.
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 crme frache each, and top with a spoonful each of lentils and caramelized onion.
