1. Prepare a medium-hot grill or heat a cast-iron skillet over medium-high heat. Rub the corn lightly with canola oil and grill, turning frequently, until lightly colored, 10 to 15 minutes. Remove from the heat and allow to cool slightly, then slice off the kernels and set aside. Don’t throw away the cobs.
2. Heat 2 tablespoons of the olive oil over medium heat in a wide, deep, lidded frying pan or Dutch oven and add the onion, carrot, and celery. Cook gently, stirring often, until the vegetables begin to soften, about 5 minutes. Add the garlic, bacon, parsley sprig, and tarragon. Continue to cook until the mixture is soft, another 5 to 10 minutes, and add the remaining tablespoon of olive oil, the potatoes, the stripped corncobs, and 1 teaspoon salt. Cook 5 minutes, stirring a few times, and add the chicken stock or water. Bring to a simmer, cover, and simmer 5 minutes. Remove the corncobs and the herb sprigs, add the green beans, cover and cook 5 to 8 minutes, until the beans are tender. Stir in the corn kernels.
3. Lay the clams or mussels on top of the vegetables. Cover and cook until they open up, 4 to 5 minutes. When the bivalves have opened, use tongs to remove them to a bowl. Return the ragout to the heat, and stir in the butter, salt and pepper to taste, lemon juice, and minced parsley. Taste and adjust the seasonings.
4. Spoon about three-fourths of the ragout onto a very large platter or into a wide bowl. Toss the clams or mussels with the remaining ragout and place them over the ragout on the platter. Scrape any liquid and ragout remaining in the pan over the top and serve.
NOTE: You can make this through step 2 several hours ahead of serving. Cover and set aside. Shortly before serving, bring back to a simmer and proves with step 3. If you have leftovers, remove the clams and mussels from the shells and add to the ragout. Reheat gently in a wide pan.