Rinse the grape leaves in a colander, then place the leaves in a bowl filled with cold water and baking soda. Swish the leaves around for a few minutes, then rinse. Place the leaves in a medium pot filled with boiling water and par-boil the leaves for 2 to 3 minutes; then carefully remove with a slotted spoon. Place the leaves on a towel or paper towel and pat dry. Remove any stems that are sticking out (they should be easy to pluck away). Set aside.

Heat two tablespoons of the olive oil in a medium skillet and saute the vegetables for 4 to 5 minutes or until just tender. Meanwhile, put the rice in a large bowl and rinse it with cold water, draining, and repeating until the water runs clear. To the drained rice, add the sauteed vegetables, lentils, 2/3 of the lemon juice, salt, pepper, and thyme, and mix.

Lay out a single grape leaf on a board. Place one heaping tablespoon of filling in the middle of the leaf. Fold each side of the leaf over the filling. Then raise the base of the leave and roll upward, tightly encasing the filling. You should have a small rolled grape leaf that looks like a burrito. Repeat, leaving 5 to 10 grape leaves unrolled.

Place a layer of unrolled grape leaves at the bottom of a heavy pan. Line up the stuffed grape leaves at the bottom of the pan, placing them close together without gaps, stacking if necessary. Top with the remaining unrolled grape leaves. Cover with cold water, remaining 1 tablespoon of olive oil, and remaining lemon juice. Cover and bring to a boil, then reduce heat to simmer and cook for 35-40 minutes, or until rice is tender. Serve cold or warm.