ABSTRACT Pulse x cereal mixtures are promoted to stabilise yield and reduce fertiliser dependence, yet robust multienvironment evidence for narrow‐leaved lupin (NLL, Lupinus angustifolius ) × oat ( Avena sativa ) mixed cropping remains scarce. We tested whether NLL × oat mixtures maintain productivity while enhancing grain‐yield stability. A randomised complete block design (four blocks) evaluated three NLL and three oat varieties grown in pure stands and as all nine pairwise mixtures with a replacement design (NLL 90%, oat 10% of pure‐stand density). The experiments ran for 13 environment‐years across five European locations. Productivity was summarised with land equivalent ratio (LER) and crop performance ratio (CPR). Yield stability was assessed using (i) the coefficient of variation (CV), (ii) Finlay–Wilkinson regression slope b 1 and Eberhart–Russell deviation variance (S 2 di ), and (iii) additive main effects and multiplicative interaction‐derived weighted average of absolute scores (AMMI‐WAAS). Mixed‐effects models were used to assess mixed cropping effects. Across environments, LER did not differ from unity (LER = 1) whether fertilised or unfertilised oat monocultures were the reference. By contrast, total CPR and oat‐specific CPR consistently exceeded 1 (mean CPR = 5.3; CPR_N = 4.6), indicating system‐level gains driven by the oat component without penalising NLL. While CVs did not separate treatments, many mixtures and pure stands were environmentally responsive, but the most stable mixtures combined near‐average responsiveness with very low variation and sensitivity to the environmental conditions (Bison × Probor and Troll × Jowisz). Lastly, fertilised oat monocultures were more stable than unfertilised counterparts. Stability gains arose from reduced environmental sensitivity of the mixture rather than the individual variety, underscoring the need to evaluate mixtures directly rather than inferring stability from monocultures. Our results support deploying pulse‐centric cereal × legume mixtures as a pragmatic option to buffer climate variability while sustaining protein‐rich production.