Onslow was always wealthy and did not need critical or financial support. The critic François-Joseph Fétis noted that, despite his absence of training, Onslow "had all the leisure necessary to overcome these obstacles". Onslow learnt to play the cello, and to play the chamber music of Mozart, Haydn and Beethoven with other local amateurs. However, aware of the need to develop his technical musical skills, in 1808 he began to study composition with Anton Reicha in Paris. At this time he also married a French heiress, Charlotte Françoise Delphine de Fontanges, by whom he was to have three children. Onslow based himself near Clermont-Ferrand, initially at his father's Château de Chalendrat at Mirefleurs (where at the age of six he had planted many of the trees), later at Château de Bellerives at Perignat, La Roche-Noire. He typically visited Paris during the winter (concert) season, when his works were often performed by musicians including the violinists Pierre Baillot and Théophile Tilmant, and the brothers Dancla, who gave quartet concerts. In 1824 and 1827 his first two operas, L'Alcalde de la vega and Le colporteur, were premiered at the Théâtre Feydeau in Paris under the auspices of the Opéra-Comique. Le colporteur was also produced in Germany, and even (in a very mangled version, in 1831), in London. In 1825 in Paris he met the 16-year-old Felix Mendelssohn, who enjoyed a performance of one of Onslow's quartets but was surprised that he was not aware of Beethoven's opera Fidelio. Onslow was an early enthusiast of the music of Hector Berlioz, whose Eight scenes from Faust (1829) and overture Les francs-juges (1830) he praised. In 1829, after Onslow had commenced his quintet op. 38 (his fifteenth), he was very seriously wounded in a hunting accident, which left him partially deaf in one ear; completing the quintet in the aftermath, he named the final movements "Fever", "Convalescence" and "Recovery". The work was subtitled "De la Balle" ("The Bullet"). Throughout the 1820s, Onslow's reputation continued to grow both in France and abroad as a series of trios, quartets and quintets were published. Onslow's publishers in Paris were Ignaz and Camille Pleyel. In 1818 his works began to be published in Germany by Breitkopf und Härtel and in Austria by C. F. Peters; the same year saw the first writings about his works by German music critics. Other German publishers, including Hoffmeister, Steiner and Simrock, followed in