## 1 Introduction  
Quercetin, with the molecular formula C15H10O7 and molecular weight of 302.236 g/mol, is known as a natural flavonoid (polyphenolic group) in various parts of onions (_Allium cpa_ L.). In addition to quercetin's anti-inflammatory and antibacterial properties [1; 2], flavonoids, renowned for their antioxidant properties, showcase a broad spectrum of biological activities and anticancer effects [3; 4; 5; 6; 7]. As an industrial waste, onion skin is produced from different food processing plants and culinary services [8]. Although a waste, onion skin contains significant amounts of valuable compounds, including natural flavonoids such as quercetin and kaempfer [9]. Among different varieties of onion, red and yellow have the major flavonoid contents [10]. In addition, studies have shown that the onion's outer layer (skin) has a higher quercetin content than the inner and middle layers [11]. Several methods exist for retrieving quercetin in the context of addressing cancer. The introduction of a microfluidic device customized for point-of-care applications, particularly focused on acoustic blood cell separation, presents a promising avenue with the capability to improve the precision and effectiveness of anticancer treatments. This innovative approach guarantees the effective isolation of therapeutic agents and cancer cells from blood samples [12; 13]. Among the various techniques for quercetin extraction, the integration of this microfluidic device signifies a notable advancement, underscoring the evolving role of quercetin in influencing and advancing strategies for cancer treatment [14]. Conventional solvent extraction methods suffer from deficiencies such as high solvent consumption, long extraction time, and thermal degradation of some sensitive compounds. Hence modern extraction methods have been utilized recently to recover various compounds from plant materials [15]. For the recovery of the flavonoid compounds like quercetin from onion skin, various extraction techniques such as subcritical fluid extraction (SFE) [16], conventional solvent extraction [17], microwave-assisted extraction (MAE), and ultrasound-assisted extraction (UAE) [18] have been employed. However, the reported extraction yield of flavonoids using SFE and MAE is relatively low [16; 18]. Among the advanced extraction methods, UAE and MAE have always proved to be efficient and eco-friendly methods due to their short extraction time and low solvent consumption for the recovery of flavonoids [19; 20]. In addition to these modern techniques, traditional methods such as maceration (MAC) and Soxhlet extraction (SOX) are commonly used, although they often involve longer extraction times and higher energy consumption. MAC is a passive, solvent-based process, while SOX provides better yields but requires continuous heating, making them less efficient compared to newer techniques like UAE and MAE [15; 16; 17]. Ultrasound-assisted extraction employs ultrasonic waves to disrupt the cell wall by creating cavitation bubbles. These bubbles grow and explode, which facilitates the process of solvent diffusion into the plant cell and enhances the extraction of the targeted compound [21; 22]. An advanced method is needed to extract flavonoids rapidly and effectively from onion skin wastes. While the combination of MAE and UAE as a sequential technique is one of the most efficient and speedy extraction methods [23], to the best of our knowledge, no previous studies have been conducted on the use of sequential microwave/ultrasound-assisted extraction (MUAE) for the recovery of quercetin from onion skin. To date, several research studies have reported the extraction of valuable products from plant materials using UAE and MAE as a sequential technique such as stage by-products for antioxidant recovery [24], flavones from _Vaccinium bracetaum_ Thunb, leaves [25], natural colorants from sorghum husk [26], and Piperine from Black Pepper [27].