Soak the Römertopf (Roman clay pot) in water for 30 minutes. Rinse the poulard inside and out. Season generously inside and outside with salt and pepper. Peel and coarsely chop the onion and garlic clove. Rinse the fresh herbs and pat dry.
Fill the poulard cavity with the chopped onion, garlic, rosemary sprig, and a few thyme sprigs. Pluck the leaves from the remaining herbs (marjoram and oregano). Gently lift the chicken skin on the breast and slide the herb leaves underneath.
Clean and cut the yellow bell peppers and zucchini into large chunks. Layer them in the Römertopf with the peeled shallots and chunky tomatoes. Season with salt and pepper. Set the stuffed poulard on top. Cover with the lid and place on the lowest oven rack. Bake at 220°C (428°F) with top and bottom heat for 70–80 minutes.
Mix the butter with the paprika powder. Remove the pot lid, brush the poulard with the paprika butter, and bake uncovered for 20–30 minutes at 220°C (428°F) until the skin is golden and crispy. The poulard is done when the internal temperature at the thickest part of the thigh reaches 74°C (165°F). Season the vegetables to taste with salt and pepper before serving.