1. Cook carrot and onion in butter in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until golden.

2. Add flour and cook over moderately low heat,
stirring constantly, until medium brown.

3. Add hot stock in a fast stream, whisking constantly to prevent lumps, then add tomato purée, garlic, celery, peppercorns, and bay leaf and bring to a boil, stirring. Reduce heat and cook at a bare simmer, uncovered, stirring occasionally, until reduced to about 3 cups, about 45 minutes.

4. Pour sauce through a fine mesh sieve into a bowl, discarding solids.