Preparation Cook carrot and onion in butter in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until golden, 7 to 8 minutes. Add flour and cook roux over moderately low heat, stirring constantly, until medium brown, 6 to 10 minutes. 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. Pour sauce through a fine-mesh sieve into a bowl, discarding solids. *Available at some specialty foods shops and cooking.com (stock requires a dilution ratio of 1:16; 1/4 cup concentrate to 4 cups water). Cooks' note:Sauce can be made 1 day ahead and cooled completely, uncovered, then chilled, covered.