Heat the poaching liquid. Bring the stock, bay leaf, celery and salt to a bare simmer in a medium pot, with tiny bubbles breaking now and then and no rolling boil.
Poach the bison. Lower in the meat and keep the heat gentle. Start checking at about 12 minutes. Pull it when a thermometer in the thickest part reads 63°C (145°F). The time depends on thickness, so trust the thermometer.
Rest the meat. Move it to a plate and rest for 5 minutes, then slice thinly against the grain. Tent loosely.
Strain the stock. Pour the poaching liquid through a fine sieve and measure out 750 ml (3 cups). Skim off any foam.
Start the base. Melt the butter in a clean pot over medium heat. Cook the onion for about 5 minutes, until soft and translucent, then add the garlic for 1 minute.
Add rice and stock. Stir in the rice and the 750 ml of stock. Simmer gently for about 12 minutes, until the rice is very soft when pressed with a spoon.
Add the beans. Add the green beans and simmer 8 to 10 minutes, until they are tender and still bright green.
Blend. Purée in batches until completely smooth, filling the blender no more than halfway because hot liquid expands. Press the purée through a fine sieve into the clean pot.
Finish. Stir in the cream and lemon juice. Warm over low heat until steaming, without boiling. Season with salt and pepper.
Serve. Ladle into warm bowls and fan the bison slices on top with a crack of pepper.